Max Tegmark, a professor at MIT, explains his understanding of the multiverse. At the first level, we can understand that beyond our observable universe, there is a planet similar to Earth and which has a person that looks like each one of us. But, due to quantum fluctuation, that person may have a slight difference, such as speaking a different language or living in another country.
The second and third levels get a bit more complicated to explain in this post. At the fourth level, he speaks of a multiverse that is based on mathematical principles. For details, take a look at this clip: https://www.youtube.com/watch?v=7kcTTBjJvME https://www.youtube.com/watch?v=7kcTTBjJvME
