OK so got a basic gist of ActivityPub I think… Objects are things like notes. Activities are things like creating or liking these objects. Actors post messages to or get messages from inboxes and outboxes to send these activities around. There’s a client-to-server part for actors to send messages to their server. And a server-to-server part to federate and deliver messages between actors on different servers.

Plenty more to it than that but enough to get by with!