I’d explain it like this:

Social media are built on ‘protocols’. Protocols tell e.g. social media what they can do and how to ‘talk’ to each other.

The Fediverse is a group of social media that use ActivityPub, Diaspora, or AT Protocol. These protocols allow something special that ‘traditional’ social media like Facebook and Instagram don’t: they can communicate across each other, without using a centralised server for hosting content.

It’s comparable to email; you can mail to someone not using your mail provider, and vice versa.

---

On one of these fediverse social media, people self-host or join a self-hosted group. Such a group is called an ‘instance’. Each instance functions independently and can have its own policies.

Instances (and users) can decide with which other instances they allow their own content to be seen. They can also decide what instances their users can see content from. An instance that is connected to another instance is said to be ‘federated’ with the latter. If that is not the case, they are ‘defederated’.

Within each instances, there are many communities. There’s a community for Linux, a community for cat pictures, a community for nature, and so on.

---

View the fediverse like a few forests, linked by many wild bridges. Lemmy might be one forest, Peertube and Mastodon yet other ones.

An instance is like a single tree. And a community a branch. Users are leaves.

Some parts of the fediverse allow these leaves to leave and join another tree.

Traditional social media, on the other hand, are comparable to a single, isolated and big tree, far away from other trees. You cannot jump to other trees, cannot easily go to a forest.

---

Would that be a good explanation?