I'm assuming what you meant is "the project page doesn't explain those" rather than it doesn't "mention" those.
And it's true that it doesn't give a definition; presumably at this earlier stage, the people most interested in the website are people who are already interested in some way in the federated social web. The ActivityPub spec is linked to though, so it's not hard to find out more info about it.
How about some high level overview of what they're trying to do instead of a spec? Not going to read the spec based on 'you're lazy, go read the spec'.
And it's true that it doesn't give a definition; presumably at this earlier stage, the people most interested in the website are people who are already interested in some way in the federated social web. The ActivityPub spec is linked to though, so it's not hard to find out more info about it.