Lemmy is a federated link aggregator and forum. Kind of like a hybrid between email and Reddit. I’m a member of Lemmy.zip, but I’m posting on another Lemmy instance (I forget where this post is, Lemmy.world, right?). Lemmy.zip and lemmy.world are “federated”, which means if users on one instance interact with users on another, both servers will sync this activity. Lemmy.world will accept lemmy.zip user posts.
Community searching shows the community name and the server where it’s hosted. Even though I only have an account on Lemmy.zip, I can subscribe, comment, and post on communities from other instances, as long as lemmy.zip is federated with them.
Recently, Beehaw de-federated from much of the fedi-verse. This means their software works the same, but prevents their users from interacting with the rest of the community, and the rest of the community from interacting with their communities and users.
It’s complicated and annoying, but necessary to be federated to prevent the fate of Digg and Reddit.
Also, one instance could require email and 2FA to be safe, and choose to de-federate from an instance that has no verification and becomes full of spammers. Or, someone could create a Lemmy instance that requires verification of identity (like AMA used to do, or the old Twitter checkmark), so if John.Oliver from the “Lemmy.OnePercent” instance posts, you know it’s the real John Oliver. There’s benefits and complications from federation.