This is what I like about the fediverse. I asked a similar question on Reddit a long time ago. Got a snarky comment "it is what it sounds like" and nothing else.
I also want to know. I notice "hot" seems almost completely useless right now. A lot of posts with hardly any upvotes or comments get counted as "hot". Yet, I believe it's the default sort, which is a shame.
I'm not totally sure but I think active is based on the rate of comments and hot is based on the rate of upvotes + random sprinkling of years old stuff.