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.
↑James, William 1895. The Will to Believe, and earlier in 1895, as cited in OED's new 2003 entry for "multiverse": 1895 W. JAMES in International Journal of Ethics6 p10. "Visible nature is all plasticity and indifference, a multiverse, as one might call it, and not a universe".