It's something I'm trying to understand too - what concepts does ActivityPub embody. My current version is "the general concept of someone posting some information that other people might be interested in, and the ability to get updates about it if you're interested". Fundamentally it's what "blog+RSS" gets you.

I haven't had a chance to go through the spec properly, but all of the other bits like liking, replies etc. seem to be more application specific.