Just to quote Wikipedia as it reflects what fediverse means quite well.

https://en.wikipedia.org/wiki/Fediverse

> The Fediverse (a portmanteau of "federation" and "universe") is an ensemble of federated (i.e. interconnected) servers that are used for web publishing (i.e. social networking, microblogging, blogging, or websites) and file hosting, but which, while independently hosted, can communicate with each other.

Usually they talk about ActivityPub, as a growing number of services are based on this (or similar).