The multiverse is like... sometimes when you make a decision, things can happen different ways depending what you choose. When that happens everything sort of splits in two. In one timeline you did one thing and in the other you did the other. After a while there ends up being millions of different timelines where history happened in all kinds of different ways, like if all the books in the fiction section of the library were talking about real places. Some of them are almost the same, some are more different, and some have almost nothing in common. Right now it's kind of like we're in the library, but we can't get inside any of the books.