I'm no expert, but I'll give it a go. On the quantum level, every interaction has a certain probability to do this, or do that instead.
According to some theories, whenever this occurs, the entire universe forks, one in which this happened, and one in which that happened. This multitude of universes is the multiverse. We live on an ever-forking path which is our reality; many others exist, some quite similar due to forking recently, some very different due to forking long ago.
Holy fricken frick. Okay, so you're saying 'every interaction has a certain probability to do this, or do that instead'. So you're talking about how at like, the sub-atomic level and shit, particles and electrons are buzzing around essentially at random, and that's what creates the forks? Or does it go deeper/smaller than that?
Again, no expert, but - my understanding is that the quantum world doesn't actually pick an outcome unless it has to - unless there is an observer, to "collapse the wave function". So the universe doesn't split unless it's forced to (so to speak) by someone/something making a quantum observation. An example of such a thing would be the observations made in the famous double-slit experiment - if there's no observer, the universe seems to keep its options open. That's a gross over-simplification, though (and some heavy anthropomorphism, to boot).
Honestly I find multiverse theory somewhat stupid. There is zero proof of it at all. It's a thought experiment. It makes as much sense to me as believing in God. It's humans attempting to answer the same questions our ancestors asked which led them to invent religion. It adds little value except to make us sleep better at night. In fact, no. Scratch that. Believing in God makes more sense to me than believing in a multiverse because at least a belief in God actually has impacts on one's life. The multiverse is vain imaginings. And it still doesn't answer the question of why there is something forking its way through whatever it's in in the first place. It doesn't answer the question of why the world exists. It just says there are lots of them.
Makes no sense? I'll just leave this here. It's pretty compelling to me.
What about the scientists who first insisted the world wasn't flat? What about the first people to say, no, the sun does NOT revolve around the Earth? That we aren't the center of the galaxy? To the people who found out we're just in A galaxy, which is one of many? To me, those things added value, and while thought experiments at the time, they were all proven to be true through science. So here we are thinking we're living in the one and only universe that's ever existed? I find that dangerously in line with the thinking of anti-heliocentrists from thousands of years ago. I'm not saying there is or is not a multiverse, but all science and history points to "we are not special, and would be naive to think this is the one and only universe there ever has been, is, and will be." We haven't proven it yet, and proving it might be beyond our perceptions and scientific tools, but it seems silly to be so sure of something that, odds are, seems likely. But with every other single step of our history, we've only proven to ourselves that we aren't special, our planet isn't unique, our solar system isn't unique, our galaxy isn't unique, and maybe someday they will prove our universe isn't unique either. I agree with that. We have a pretty good idea how our universe started, the big bang, but that still doesn't tell us anything about what happened before that, or what that matter came from, or what was here before. It raises more questions than it answers, and so would the discovery of a multiverse. And I take a small personal offense to comparing the multiverse possibility to believing in God, and saying that god actually impacts ones life. For some of us, space impacts our life for the same reason God impacts other. Some think God created them? Well, the universe created me, and the more we learn about it and discover, the closer we get to ACTUALLY discovering our origins of true creation, and the answer to why we are here. To me, space does impact my life, far more than any God could.It adds little value except to make us sleep better at night.
It doesn't answer the question of why the world exists. It just says there are lots of them.
There's a lot more background here : https://en.wikipedia.org/wiki/Many-worlds_interpretation It's not meant to be taken literally - no one thinks the universe really copies itself, when the wave function collapses. It is exactly a thought experiment - of a similar kind to the ones which lead Einstein to relativity. Doesn't make it right or wrong. However, my guess is that we WILL need to have new ways of thinking about events and time, if we are to ever understand quantum reality.