I don't think that anyone understands them. The frustrating thing is that if there are parallel universes there is, as far as anyone yet knows, no way to know if they are there at all. The reason people think that they might exist is based on the theory of black holes. It just so happens that if you play with the math used to "derive" black holes you can see that it makes sense to talk about a place just like our universe "on the other side" of the black hole. What do you think about that? I guess that the question is, if something can exist mathematically does it really exist?