See Mastodon, Peertube, Pixelfed etc.

The Fediverse is a system where social networks are hosted on many servers worldwide that aren't part of any central company but instead hosted by individuals. But these servers can connect to other such servers. So a Fediverse is a federated network. It is decentral on one side, but server owners can decide to enforce own rules on their server (also called an instance) and even block other servers. So it's more decentral than, for example, Twitter, but not as decentral as Nostr or Bitcoin.