ActivityPub is a social networking protocol. Think of it as a language that describes social networks: the nouns are users and posts, and the verbs are like, follow, share, create… ActivityPub gives applications a shared vocabulary that they can use to communicate with each other. If a server implements ActivityPub, it can publish posts that any other server that implements ActivityPub knows how to share, like and reply to. It can also share, like, or reply to posts from other servers that speak ActivityPub on behalf of its users.