So here's a scenario: You've signed up for a particular social media website, and after about ten whole minutes of fucking around and enjoying yourself, you've run into someone that, by your logic, shouldn't exist. They're an alien that talks about space travel beyond known capabilities, or a mythical creature that talks about magic, or a fictional character you know of, or a version of someone you know or have known that's fundamentally different from the one you've seen. It's likely that, within a few moment of trying to talk to this kind of person, they've tried to explain the concept of parallel realities, or alternate timelines, or multiple universes, or any other combo of those terms. This page is an attempt to be a comprehensive layman's guide to that idea as it applies practically to people's lives within it.
When people try to explain the concept of parallel timelines and universes, they usually go for the example of choosing between two things to eat. For the sake of easier relatability, we're going to say your choices are Option A or Option B. We're also not going to factor in any other variables that might make you pick one over the other, which makes this essentially a random, 50/50 shot between the two.
From your perception, you simply pick one and move forward with your life. In actuality, though, both of these choices occur at the same time. Reality itself has split, and your perception has remained in the version resulting from the action you took. This happens every time you make a choice, and has since you started to exist. It's also happened every time everyone else has made a choice, for the existence of your species, and any species that has lived concurrently or before them, since the dawn of time.
You might think that sounds overwhelming. You might also think that sounds kind of fucking stupid, since minute choices usually don't have noticeable cosmic-level consequences. Both of these observations are true, since this is around the point where the example starts to outlive its usefulness. In theory, every tiny action that is taken or event that can happen results in another version of reality, creating a functionally infinite amount of branching timelines. In practice, however, it's extremely likely that you will only ever deal with timelines that have had a more drastic event separating it from other potential realities. Time splits off like an infinite tree, but most people only deal with the larger branches.
For example, let's say that you have Option A and Option B as choices for food again, but this time I hate you and have dosed Option B with a lethal poison. Still assuming that there's no other factors in play and there's a random, 50% chance of choosing either one, there's now two equally likely timelines: One in which you live, and one in which you die. Regardless of who you are, this makes a notable difference, since no events that involve you as a living person can happen if you are dead, and vice versa. This is the key to noticeable timeline splits: creating exclusive events that can only happen because of the actions further up the stream. Said action might be much more mundane, but the death of an average person is roughly the minimum level of difference seen between timelines that an average mortal person can observe and interact with.
Q: Wow, so [INSERT LOCAL ANOMALY] is actually real?
Q: Will something explode if I see/touch/kill an alternate verison of me?
Q: How come magic is "real" in some universes, but not others?
A: I'm not a scientist.
Q: This has some weird moral implications, doesn't it?
A: That's not my department either.
Q: Can people from these other universes visit me?
A: With strong enough magic or tech, yes.
Q: Can someone from another universe visit without my knowledge?
A: Most kinds of multiversal travel need some level of participation from someone at the other end. But not all of them. So watch out for that.
Q: That sounds scary. How can I protect my timeline?
A: Ask around for an expert.
As you observe the universe, you might notice that there are a lot of events that seem to happen regardless of how other circumstances have changed.
In another weird example of multiversal patterns, you might find people online who are, in your universe, a fictional character. You might also discover that in other universes, you are a fictional character.
Q: I found someone who's a fictional character in my universe. Do they know they're fictional sometimes?
A: Usually, the average person won't know unless someone has already told them.
Q: Should I tell them?
A: Most people don't take it very well, and it can have a lot of social consequences. I'm not stopping you, though.
Q: How do I stop magic anons from happening?
A: Turn off anon. They're cowards.
Q: How do I stop a magic anon that's already happened?
A: Ask someone who's good at magic nicely.