The Federation, or as often called the "Fediverse", refers to a global social network composed of nodes that talk to each other. Each of them is an installation of software which supports one of the federated social web protocols.

the federation statitics hub

The fediverse is a series of social networks that are able to communicate with one another using shared protocols. It uses a few different protocols over the different services, but ActivityPub (and its flagship platform Mastodon) is the most popular.