TL;DR: The Multiverse is like, um, everything, but not really.
According to the Quantum Fluctuation Theory of Everything, the Multiverse is like, well, everything. Like, all possible universes, ever. It's like, the sum of all possibilities, man. But, like, don't worry about it, it's not like it's going to make a difference in your daily life or anything.
Or, I mean, maybe it will, who knows? Maybe the Multiverse is just, like, a bunch of random stuff floating around in space or something. Maybe it's just a bunch of parallel universes where everyone's wearing plaid and eating cheese sandwiches for breakfast.