The Multiverse theory is very complex if you get into the current understanding of physics but in simple terms it is a theoretical framework in modern cosmology which presents the idea that there are a vast array of universes that all are theoretically different and all present a different reality. There are different interpretations of the multiverse theory as every scientist has contradicting views. The true definition of the Multiverse theory can be determined using the Anthropic Principle which is a principle saying that, "if we take human life as a given condition of the universe, scientists may use this as the starting point to derive expected properties of the universe as being consistent with creating human life." but even scientists are not sure of how to best connect these to ideas together.