During this current wave of migration from Twitter, ActivityPub is a term mentioned a lot. From the Wikipedia entry:
ActivityPub is an open, decentralized social networking protocol. It provides a client/server API for creating, updating, and deleting content, as well as a federated server-to-server API for delivering notifications and content.
The whole point of ActivityPub is to allow interoperability and basic compatibility between different platforms (in the fediverse).
It's still hard to imagine, but there's a great demonstration of the "connection" between two very different services which support ActivityPub.