You're thinking of parallel or alternate universes, which is based on the "many worlds" interpretation of quantum mechanics, where the universe "forks" at every

You're thinking of parallel or alternate universes, which is based on the "many worlds" interpretation of quantum mechanics, where the universe "forks" at every quantum possibility into an infinite number of universes. That's one type of multiverse. A "Multiverse" is a superset of that idea—it simply means a condition of many universes existing. They could have arisen in the same way ours did through a "big bang" in some larger space alongside our universe, they could be the many iterations of our one universe expanding and contracting over an infinite time, or they could be overlapping in the "many worlds" sense; we don't know. The point of a Multiverse set is to explain our universe's existence statistically rather than empirically (as in, with a satisfying observational explanation). It says "we know that the chance of our universe existing is extremely small, so we must be one in an infinite number that had the right combination to result in life." This is sort of like a car crash, and saying: "whelp, it happens with a probability of 1 per 1000, so it was bound to happen to someone" after a car accident rather than finding out exactly why and what caused the event—"whelp, the brakes didn't work, we should probably look into that." Just like that car crash, finding out exactly why our universe exists, and how likely it is to have the reality we enjoy, gives us far more information than a simple philosophical statistical conclusion. It doesn't mean that the statistical conclusion isn't correct—there still may be a Multiverse, we may never know—it just means we know more about it than just how often it happens. But, knowing how likely our universe is to happen is a big step. If it's very likely, then we know a lot more about universes. If it's unlikely, then we could just chalk up our existence to chance and it would be perfectly valid. In the latter case, however, it would be extremely unsatisfying scientifically. We'd love to know more.