Are Many Worlds and the Multiverse the Same Idea? - When particle physicists say “vacuum,” they don’t mean “empty space,” they mean “a state of a theory that has the lowest energy of all similar-looking states.” So let’s say you have some scalar field filling the universe that can take on different values, and each different value has a different potential energy associated with it. In the course of normal evolution the field wants to settle down to a minimum of its potential energy.
Decay of the False Vacuum - The Astronomy Cafe, Dr. Sten Odenwald
False Vacuum: (A) scientific hypothesis that our universe is actually in a false phase state as part of a larger universe, like if it were a temporary thing (think the real universe is a pot of boiling water, and we are just within a bubble forming at the bottom of the pot). Eventually however that false vacuum has to pop, even after billions of years in this false state and we and everything we know in our visible universe will disappear in an instant with no warning whatsoever