A federated social networking protocol.
In a way ActivityPub is very similar to eMail. Users are registered to respective servers and messages are passed from user to user via the servers that communicate with each other. Whereas eMail just passes on unstructured (plain text) messages, ActivityPub passes on structured content (RDF).
Currently ActivityPub is mostly used for microblogging (sharing simple notes). There are other uses (images, events, links) but the full potential of what kind of data can be created and shared via ActivityPub is not used.
openEngiadina aims to use ActivityPub for arbitrary structured content by using the Semantic Web aspect of ActivityPub.
The network of all servers (and clients) communicating over ActivityPub is called the Fediverse.
Projects using ActivityPub
See also Inspiration and related projects for more projects.
The W3C published standard.
Overview of the Fediverse and statistics of servers.