In a series, typically the protagonists or the setting or some characters are the same – so they are part of the same fictional universe – but each book tells a separate, standalone story that can be read on it's own, with either no overarching storyline or only one that develops slowly and isn't driving the main plot of each book. A series is open-ended and can continue indefinitely.
In a multivolume work there is one main story arc that develops over several books. None of the books can be read on their own because significant parts of the story will be missing. A multivolume work consists of a predifined number of books and ends after the last one.
If you write a series, make sure each book stands on its own and none of the others must be read to understand an individual book from the series and the order of reading them is mostly irrelevant. Think of the Reacher series by Lee Child.
If you write a series, make sure you don't give away so much of the plot of the other books that reading them will feel like "I've read this before".
If you write a multivolume work, they may or may not have a specific order of reading them. If there is some kind of progress or revelation occuring over the books, they have a specific reading order. If they narrate the same events from the contradicting perspectives of different characters and none of the views is eventually revealed as the correct one but they remain equivalent, the reading order is irrelevant.
You decide what kind of story you tell. Is it a progression from some beginning toward some end? Then you may have to leave out certain information that is revealed in later books from the earlier ones. Or is it achronological and each volume stands "beside" all others? Then make sure that the overlapping information is either small (so as not to bore the reader) or conflicting (so as to intrigue the reader and irritate them) or new (that is, every viewpoint provides a different part of the puzzle and you need them all to understand the whole).
