WebFinger
The protocol that makes @name@server.com resolvable. Standardized in RFC 7033. Without it, fediverse addresses would be URLs instead of something resembling an email address.
The mechanics are deliberately trivial: an HTTPS request to https://server.com/.well-known/webfinger?resource=acct:name@server.com returns a small JSON document with links to the account's actual resources — the profile page and, crucially, the ActivityPub actor URL. From there follows and posts can be handled. The name is a joke about the Unix finger command, which in the 1970s showed who was logged in and when they last read their mail. It's used by Mastodon, Misskey, Lemmy and the rest of the ecosystem, as well as by some OpenID and Matrix implementations for finding the right server.