The Wikipedia entry does a very good job in its first few paragraphs:

The fediverse (a portmanteau of "federation" and "universe") is an ensemble of social networks which can communicate with each other, while remaining independent platforms. Users on different social networks and websites can send and receive updates from others across the network.

ActivityPub, a W3C standard, is the most widely used protocol that powers the fediverse. Noted fediverse platforms include Mastodon, Lemmy, PeerTube, Pixelfed, and Threads.