What is the Multiverse?
Using the standard definition of “universe”, there can’t be more than one universe because any newly discovered universe would simply be considered a previously-unknown part of the Universe.
However we could assume that our Universe consists of all the space and time within it, but any places that exist independently of our definition of space and time are separate universes. Together they would form the “Multiverse”. Other universes could be like ours or they might have completely different laws of physics. They wouldn’t exist “far away” in the sense that we normally think of distance. They would exist in some kind of parallel dimensions completely seperate to what we think of as space and time.
We have no evidence that other universes exist—they’re entirely hypothetical. Will we ever know if they do exist? It’s a long-shot but there is a slim chance that “external universes” might leave an imprint somewhere in our own Universe. If that’s the case we might one day be able to find them. Until then, most astronomers work on the assumption that there is only one Universe.