The physics is inspired by the fact that Quantum mechanics is "reversible" if left unobserved (that is, given the state of the system at any one point, you have enough information to work out the Quantum state at any previous or future point in time) but when measured or observed results in a wave-function collapse, and loss of some information means that there is no longer enough information in the system to recreate the Quantum state as it was before measurement. Observation of a Quantum system from an "external perspective" may turn out to be impossible though, as the observer must form part of the complete system. So as a whole, the "Multiverse" must be be reversible, and we experience only part of it. Mathematically this means that if the outcome of an experiment could be one of two results, the Multiverse has to keep track of both for reversibility...
In many Fantasy and SciFi settings, the word for all the universes that the characters can travel between.