@thatonekidd5 this is the short definition: https://github.com/tootsuite/documentation/blob/master/Using-Mastodon/FAQ.md#what-is-an-instance

So, I'm on computerfairi.es and you're on mastodon.social and these are both instances. Many of them definitely work, even though the way "federation" (the network of all those instances) works can be a bit counter-intuitive at first and make you wonder what happens at times. But usually someone will have an explanation if you ask :)