What is the difference between the terms "active" and "selected"?

Question

I'm learning UV editing in Blender. I'm editing the UV maps like following:

Now, I want to rectify the island containing the selected quad. I expect only the selected island to be modified. But when I select the whole islands by L-key and execute Follow Active Quads, other islands are also modified like this:

I found that a quad in the leftmost island was displayed in light gray. So this quad should be active.

I understand the two islands followed each active quad. But why do two islands both have their active quad? But why does the quad stay active when I click another one?

My questions are as follows:

1. What is the difference between the terms "active" and "selected" in Blender?
2. Is there a way to deactivate a quad?

Answer 1

There can be more than one selected item. For example in your second screen shot you have six selected faces.

Of the selected items, exactly one is active. Usually it is the last item you've selected when you are selecting multiple items.

The active item plays a role in various operations, such as parenting or copying properties.

You deactive a quad in face select mode by deselecting it, or if you wish it to remain selected, adding another quad to the selection. The newly added quad will become the selected quad.