Explaining it as simply as I can manage
So Mastodon isn't a website, it's software, used by different websites to provide a twitter-esque experience (it has some differences which I'll talk about later). These websites are (by default) able to interact, so if I'm on red.corn and you're on green.corn, we can still follow each other. If you boost (retweet) a post from someone on blue.corn, I'll see that post, too.
Honestly I think for most people that's about the gist of it. Different instances (websites) have different rules, themes, and cultures, but if I'm on iLoveComputers.corn and you're on ProfessionalPainterz.corn, we're still able to follow each other (unless they've defederated from each other, but, in my experience, instances only defederate over big things, like hosting bigoted content and such).
I'll put more here over time, but I think, for most users, that's all you really need to know! There are plenty of guides to finding a good instance, but I'll share a few instances that people I follow are on. Most are furry-focused, but not all! I have not vetted any of these personally except for meow.social, which I use; as such, I make no guarantee that they are accepting new users, run well, enforce their rules consistently, etc. Some may contain adult content (I do not know and have not checked).
Hopefully this gives an idea of how you can interact with people on other instances!