Often times people ask me, "But how do I learn about how ActivityPub works? Where do I get started?"
Friends! The ActivityPub spec itself is here to help you! There's a really lovely story-driven tutorial about ActivityPub right at the top of the spec in the "Overview" section! https://www.w3.org/TR/activitypub/#Overview
Give it a read! I tried really hard to make it easy to follow, and it has beautiful illustrations from @mray!
www.w3.orgActivityPubThe ActivityPub protocol is a decentralized social networking protocol
based upon the [ActivityStreams] 2.0 data format.
It provides a client to server API for creating, updating and deleting
content, as well as a federated server to server API for delivering
notifications and content.
@mray @cwebber I’ve been waiting for federated social to come of age since https://www.w3.org/2005/Incubator/federatedsocialweb/wiki/SWAT0 
www.w3.orgSWAT0 - Federated Social Web Incubator Group