ActivityPub is a decentralized social networking protocol. It is an open standard that enables interoperability between different social media platforms and applications. ActivityPub allows users to create and share content, follow other users, and interact with their posts through comments, likes, and shares. It promotes a federated approach to social networking, where users can have their own instances (servers) and still connect and communicate with users on other instances. This protocol has been adopted by various platforms, including Mastodon, PeerTube, and Pixelfed, among others, to create a decentralized and interconnected social web.