ActivityPub is a decentralized social networking protocol that allows different platforms to communicate with each other. It enables users to share content, follow others, and interact across various services without being confined to a single platform. This means that a user on one service can connect with users on another, promoting a more open and interconnected online experience. The protocol is based on the ActivityStreams format, which structures data in a way that makes it easy to understand and share. By using ActivityPub, developers can create applications that support user interactions, such as posting updates, liking content, and sending messages, all while maintaining user control over their data.