Instances are like smaller Reddits. Communities (lemmy) and magazines (kbin) are like subs. You can subscribe and participate in any c/ or m/ as long as their instance is federated with yours.

To add to the fun, we can also interact with users on Mastodon (similar to Twitter), Pixelfed (like Instagram), or any other ActivityPub-enabled instance that federated with us.

To my mind, a better analogy is email. It doesn’t matter which platform (provider) you use, you can interact with anyone on almost any other email platform. Make sense?

a new instance doesn't know about any other instances at first. when someone on the new instance, instance A, searches up a community on instance B, instance A now knows about the existence of instance B and starts pulling content. the more communities people on instance A subscribe to, the more of the Fediverse instance A sees.

what this means in practice is that a community can be hosted on any instance, and people from any other instance (that isn't blocked by the community's instance) can post and comment on it seamlessly. the community you posted this to is hosted on lemmy.world, but you have an account on kbin.social. and i have an account on sopuli.xyz!

this means that you get the freedom to pick whichever instance you want to set up your account. since each instance is its own independent website, if one instance goes down, the rest of the network isn't affected. you could even set up your own instance on your own hardware! different instances have different rules and vibes, and if one instance is misbehaving (e.g. mods have lost control, the software is glitching and spamming other instances with too much traffic, there are nazis), other instances can block it temporarily or permanently.

as for how many instances there are, FediDB lists 1,175. no instance sees every other instance, so there isn't an optimal one to join like some people have asked about

Wrote this once so I'd never have to write it again. lol Here's how I word it:

https://lemmy.world/post/583669

You can subscribe to any community no matter what site you use.

That's the fun part, it doesn't.

Joking aside it's my understanding that instances grab new content from communities/magazines it knows about, as much as possible. From what I've been seeing, federation in both Lemmy and Kbin can be a bit wonky though, I've gone back to Lotide.