The Multiverse in short is the theory that alternate realities exist. There could be a reality where you have won the Lottery or not won if you're a winner in this reality. There's a reality where Hillary Clinton won the U.S. Election and not Donald Trump. You can go on and on with other possible realities, each one different, each one based on a different set of rules.
There is no hard evidence that the Multiverse exists, there is only theory. Should a Multiverse exist, it could explain how our universe came into being. There is a theory that the big bang was caused by two universes colliding therefore creating a third.
According to scientists at the University of Durham, Professor Tom Shanks and postgraduate student Ruari Mackenzie believe that a cold spot in the Cosmic Background Radiation is the result of a collision between two universes. Their work is published in a Royal Astronomical Society paper. The picture below is copyrighted E.S.A. and Durham University.
The cold spot on the map is colder than most of its surroundings and void of many galaxies compared to surrounding areas. The cold spot was thought to be as a result of super void but current research believe it is too big to be the result of a void and therefore the Multiverse theory has gained substance. Ref: Independent