Thats interesting.
Active prioritises posts with a high amount of activity, and keeps them close to the top for a while. This makes lemmy look more active.

https://join-lemmy.org/docs/users/03-votes-and-ranking.html This explains how the algos work.

Hot only takes NSFW posts into account. 😉

I think Active are posts that get a lot of comments and Hot are posts that get a lot of upvotes. Of course they can overlap.

For me Hot sends to be a combination of upvotes and comments. Active seems to be for newer posts with a steady increase in votes. I also seem to get more duplicates with Active compared to Hot.