The protocol describes a decentralized "social network"; it relies heavily on JSON+LD schemes to describe
activities performed by
actors. The protocol comprises a set of interfaces for
client-to-server connections. You can find more information in the W3C ActivityPub page
In the following diagram, you can see Alice and Bob interacting with the AcitityPub protocol. Alice wants to post something that could be a Tweet-like message, a video, or a note; Bob should be able to read it.
A service discovery protocol is the base of ActivityPub. The representation of all actors, Alice and Bob, and the messages have URLs in the attributes pointing to the counterpart server. The service discovery definition uses JSON+LD as the default format.
Those URLs tell where a server or a client must fulfill an activity such as Publish or Like. In the previous example, Alice wants to publish a post. She already knows the Outbox URL to create a Post; in this case, the action is "Create."
I used to be a PropTech entrepreneur. I implemented the Real Estate Transaction Standards (RETS) for the web application. The Multiple Listing Services (MLS) uses RETS to describe properties and actions performed over it. Those standards or, better, a new version may benefit from this kind of protocol.