A multiverse (or meta-universe) is the theory of a set of multiple possible universes, including our universe, which make up reality. These universes are sometimes called parallel universes. A number of different versions have been considered. They all have the same weakness: there is no known way these ideas can be tested.
You can think of universes as "bubbles". When two of these "bubbles" collide they may have caused the big bang. They can also separate to form two smaller universes.
The term "multiverse" was coined by psychologist William James.