nostr:npub1h3t28gelg2xc4j8zjulj7jz62j5cernn0aga9srvy8w9qeqwxmtsnldkep "Activity Pub" is the name of the Internet protocol that enables the fediverse to share messages between servers using different kinds of software that all support the protocol. That's why Mastodon messages can be exchange with Pixelfed or Firefish users, and vice versa. If your app speaks the protocol, it can at least potentially communicate with every other app that does. The concept is what makes decentralized social media fundamentally different from what has come before.