tuckerm , 3 hours ago

Yep, that's basically all of it. ActivityPub allows a reader to send messages back to the original poster. Those messages can be a comment, or a like, an upvote, a downvote, or a many others. That's what ActivityPub unlocks compared to RSS.

RSS only goes one way: the reader can read messages from the poster, but not send any messages to the poster.

edit: if anyone is curious about what the "many other" messages can be, the list is here, under Verbs: https://github.com/activitystreams/activity-schema/blob/master/activity-schema.md#verbs

Technically, that is part of ActivityStreams, not ActivityPub. But there is a lot of overlap there, and ActivityStreams is a necessary addition. For example, downvotes on Lemmy are not part of AP, but you'll find them in AS, called "dislike."