The Multiverse is a vast expanse of infinite universes, each with its own unique set of physical laws and properties. It's like, but not exactly like, the concept of parallel universes, but with more parallel-ness.
We're still counting. It's a big job, okay? But let's just say there are more universes than you can shake a stick at.
Sorry, no. Inter-universal travel is strictly regulated. Don't try to build a wormhole in your garage.