Lemmy is built as an alternative to Reddit. Instead of a single website, however, multiple people can each set up their own Lemmy, each of which is called a Lemmy instance, or just an instance for short. This person becomes its administrator, or administrators if it a a group of people. The special thing about Lemmy is that you’re allowed to join subreddits, called communities, in other instances of Lemmy. This intercommunication between different instances of Lemmy forms a network of instances.

But how do the instances tell each other what’s posted in their instance, such that each instance have no trouble sharing information? Since they’re all Lemmy, they just need to pick a standard way of sharing their data. That standard, which we call a protocol, is called ActivityPub.

ActivityPub isn’t a new thing, nor is it unique to Lemmy. One very popular service that uses this protocol is Mastodon, a Twitter alternative, and, like Lemmy, you have multiple instances of Mastodon that forms a network with one another through the ActivityPub protocol. Mastodon has long been calling their network the Fediverse, and with its current popularity, the word Fediverse has, for most people, become the word for any network that uses the ActivityPub protocol for intercommunication.

We won’t go into details or cover everything about what a Fediverse is, but one very important feature is that the people who set up their instances have the power to join or not join the Fediverse. Each instance can in turn block other instances, which is called a defederation. No one has the power to tell every other admin to defederate any particular instance — it’s up to the admin(s) of each instance.