What is a spell slot in-lore, and how does it justify the limits on casting spells?

Question

Spell slots are something that we, as players, expend when we want our spellcasting PCs to cast a spell. It is a resource to limit how many powerful spells we can cast in a day. But for our characters in-game, they don't exist. So what are they?

The best way I can think of to illustrate my question is via an example involving a Kenku.

Background

I was going to ask the question "Can a (non-spellcaster) Kenku cast a Verbal-component-only spell that they have heard a spellcaster cast via Mimicry?" I knew the answer would be that they can't, but I was wondering what the in-game justification is for this.

Out-of-game, the answer is that they do not have the Spellcasting (or Pact Magic) class feature, and therefore do not have spell slots to expend to cast the spell, but that just replaced one question with another; what is a spell slot in-game? Knowing the answer to this would justify why the Kenku perfectly mimicking the Verbal component of a spell doesn't work in-game.

Comparison

There are other terms we use: HP, AC, XP; these terms do not exist in-game. My PC won't know what "HP" is. HP has an in-game description, as explained further in this question: What does HP represent?

In short, from the PHB, pg. 196:

Hit points represent a combination of physical and mental durability, the will to live, and luck.

So those are things that my PC might know about and understand; luck, the will to live, etc. They make sense in-game and are something my character could talk about.

What I've Looked Up

A Wizard's Spellcasting class feature (PHB, pg. 114) only describes the mechanics of what a spell slot is to the player (I didn't check the Spellcasting class feature for all the other classes), and the Spell Slots section (PHB, pg. 201) simply says (regarding flavour):

Manipulating the fabric of magic and channeling its energy into even a simple spell is physically and mentally taxing, and higher-level spells are even more so.

But that doesn't explain why, say, a Kenku who has learned to mimic a Wizard's spell's Verbal components couldn't cast a spell (without being a spellcaster class themselves; i.e. they have no spell slots).

Sure, it might "tax" them, but surely they'd be able to pull it off at least once that day? Or is it so taxing to even say that specific word or phrase that they wouldn't actually be able to even finish saying it "without the proper training" (e.g. being a Wizard), and thus cannot "complete" the spell? (NB: This isn't my question, it's just included to show my train of thought.)

Question

So what are spell slots in-game? What in-game "thing" do they represent? Is there an in-game justification for why a character who has spell slots can cast a spell in-game, whereas a different character without spell slots could not (even if they can satisfy the spell's components; i.e. a Kenku perfectly mimicking the Verbal component)?

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If the flavour of certain spellcasting classes would influence the answer such that all classes cannot be explained by one explanation (i.e. because Warlocks have Patrons, Sorcerers have "a spark of magic within them", divine casters have gods or ideals, etc) then this question can just focus on Wizards specifically and what their spell slots mean, since a Wizard's relationship with magic (i.e. "learning") is closer to how a Kenku "learns" the Verbal component via mimicry.

Also, I'm not particularly interested in a settings-specific answer, but if a specific setting would influence an answer, let's assume the Forgotten Realms (as it is the default setting of 5e).

Just to clarify: I don't think the Kenku should be able to do this (e.g. a level 1 Kenku overhears a high level Wizard cast wish, uh... no), my question is why not from an in-game/lore perspective.

Answer 1

As you noted, according to the PHB:

Regardless of how many spells a caster knows or prepares, he or she can cast only a limited number of spells before resting. Manipulating the fabric of magic and channeling its energy into even a simple spell is physically and mentally taxing....

So therefore, spell slots are an abstraction of arcane potential: how much magic a character can cast in a given day.

"Manipulating the fabric of magic" isn't just flavor; it's the abstract description of what is happening in-game.

There is therefore something, some source of arcane ability, that each caster has that allows them to manipulate the fabric of magic, something that non-casters do not have.

For different kinds of casters, that source of arcane ability is different. For some it is divine, or an otherworldly patron, or an innate magical gift, or the forces of nature.

You mentioned wizards specifically. Wizards have developed their arcane power after years of study:

From the PHB:

Though the casting of a typical spell requires merely the utterance of a few strange words, fleeting gestures, and sometimes a pinch or clump of exotic materials, these surface components barely hint at the expertise attained after years of apprenticeship and countless hours of study.

By definition, merely the verbal, somatic, and material components are insufficient, it is the mental expertise the wizard has gained over many years.

So, while the verbal, physical, and somatic components are (usually) a necessary component of casting a spell, they are not by themselves sufficient. There is something else, something internal, something attained in one way or another, perhaps by years of mental preparation, a divine or natural connection, an otherworldly patron, or some innate ability.

That's why merely mimicking the components isn't enough.

Answer 2

Spell slots represent the caster's mental limits

D&D 5th edition's Player's Handbook, p. 201, under Spell Slots, describes them thusly:

Regardless of how many spells a caster knows or prepares, he or she can cast only a limited number of spells before resting. Manipulating the fabric of magic and channeling its energy into even a simple spell is physically and mentally taxing, and higher-level spells are even more so. ...

When a character casts a spell, he or she expends a slot of that spell's level or higher, effectively "filling" a slot with the spell. You can think of a spell slot as a groove of a certain size—small for a 1st-level slot, larger for a spell of higher level.

In the earliest editions of Dungeons & Dragons, a spell was "memorized" once, and actually disappeared from the caster's memory when cast. This comes from the stories of author Jack Vance, where magic works in a similar manner.

Under this system, a caster simply had a limit of how many spells he could memorize, with more skilled casters capable of memorizing more spells at a time. More powerful spells were harder to memorize.

In D&D 3rd edition, the lore changed significantly: casters only prepared spells, not memorized them. This means they cast most of the spell except for the final few words, which serve as the trigger. Incidentally, this is why you can't cast a spell by repeating the sounds a wizard makes: the words are merely a trigger for the prepared spells stored in the caster's mind, and unless you've actually prepared those spells, those words do nothing.

The limits to preparation were described in the third edition Player's Handbook, p. 178:

If a wizard has cast spells recently, the drain on her resources reduces her capacity to prepare new spells.

The wizard's intelligence is a limiting factor:

Preparing an arcane spell is an arduous mental task. To do so, the wizard must have an Intelligence score of at least 10 + the spell's level. ... To prepare her daily spells, a wizard must have a clear mind.

Casting a spell is also described as draining on a wizard, as fighting might be for a warrior:

Upon the casting of a spell, the spell's energy is expended and purged from the character, leaving her feeling a little tired.

It is similar for clerics, druids and paladins:

Divine casters prepare their spells in largely the same manner as wizards do, but with a few differences.

D&D 3rd edition opened up the idea of spontaneous casting with the Sorcerer class, who simply "knew" spells and cast them spontaneously using spell slots. Designer Monte Cook believed that the resulting increase in versatility actually made the class more powerful, because you are more likely to be able to cast the optimal spell in the right circumstance.

The limits of spontaneous casters were less clearly described. They have a limit because that's the best they can do until they gain a new level, but there's no discrete lore reason why.

According to those editions, spell slots represent the caster's mental stamina and capacity to hold magical energy in their mind. I recall a designer (Mearls, possibly?) saying that spell slots in a caster's mind are analogous to a quiver of arrows, which are discrete units of effect and one cannot fight when they are exhausted.

The 5th edition Player's Handbook also describes a spell slot as a mental container of magical energy, under the wizard section on p.115, under Arcane Recovery:

You have learned to regain some of your magical energy by studying your spellbook. Once per day when you finish a short rest, you can choose expended spell slots to recover.

Answer 3

This is setting-specific, which is why it is not spelled out in more detail in the rules and left as an abstraction. From Sword Coast Adventurer's Guide:

The Weave is an essential element of the universe, running through everything in unseen threads. Some creatures, objects, and locations have deep, intrinsic ties to the Weave and can perform extraordinary feats that come naturally to them (a beholder’s flight, a vampire’s charming gaze, a dragon’s breath weapon, and so forth). Creatures with the necessary talent and skill can also manipulate the Weave to perform magic by casting spells.

So, in Forgotten Realms, spells slots are some aspect of how the "Weave" is manipulated by characters or creatures with spellcasting ability. The details of how that exactly works is left to Mystra — the goddess of magic. In your example, your kenku mimic knows the sounds and motions, but does not know how to access the Weave.

I don't own the 5E Eberron material, but Keith Baker describes magic as being like a force of the universe where spellcasting is treated as a science (and slots are a consequence of the application of that science). In the post, Baker describes the possibility that magic missile was originally (in the fictitious history of the setting) a third-level spell but the application of research brought it down over time to a first level. This is less satisfying in the kenku mimic case, but... there it is.

In Dark Sun, magic draws from the lifeforce of living beings or of the planet itself. Again, spell-slots are the abstraction of how this works — and the kenku mimic is simply saying the words, not actually drawing that force.

Answer 4

As far as I'm aware, there's no canonical explanation of what spell slots are in fifth edition. In older editions, there was such an explanation, but that explanation is incompatible with fifth-edition rules.

Historically, spell slots were imaginary scrolls.

Prior to third-edition, a Mage's (they weren't called wizards, back then) spellbook was literally a collection of scrolls that the wizard had bound together. Scrolls, then as now, would allow the spell to be cast, a single time, after which they would go blank.

A "prepared spell" was the mage carefully reading the scroll and creating a copy of the scroll in their mind. When they cast the spell, the imaginary copy of the scroll in their mind would go blank.

Spell Slots, at that time, were the number of imaginary scrolls a mage could fit into their mind before they started forgetting them.

This doesn't match fifth-editions mechanics

This interpretation worked even in third edition (except that it was no longer possible for a Wizard to read directly from their spellbook), but in Fifth Edition, there was a mechanical change to the way Spell Slots worked that is not compatible with this interpretation. Since memorized spells and spell slots are now separate mechanics, this interpretation cannot work in fifth edition.

Answer 5

Mana

Each spell slot represents a set quantity of magical energy (commonly called Mana). Each spell requires the use of a set amount of a particular quality of Mana at the time of casting.

As a character gains experience, the available pool of Mana increases, and the number of spells that can be cast increases as a result.

The different levels of spell slots represent increasingly rich sources of Mana. Some spells have an increased effect when a richer Mana source is utilised for the casting (casting the spell at a different/higher level). Once again, as the character gains experience, increasingly rich sources become available.

The Kenku's lack of spellcasting ability can be explained by being unable to access a store of Mana. This may be a restriction based on profession, or possibly a hindrance suffered by the race as a whole. (I don't have any books to hand that might confirm or deny this, but I do remember Kenku being a playable race at one point, so professional limitation might be more accurate. As always, Rule 0 applies.)