Worth noting there are descriptions here: https://join-lemmy.org/docs/users/03-votes-and-ranking.html

But basically Hot is highest score (upvotes minus downvotes) with a decay based on how old the post is (down to 0 after I think 2 days)

Active is the same thing but with the decay based on when the last comment was posted rather than the post itself (use to find conversations).

Top is just the highest score (upvoted minus downvotes) only counting the votes in the period you select (hour, 6 hours, day, etc).