"Active (default): Calculates a rank based on the score and time of the latest comment, with decay over time
Hot: Like active, but uses time when the post was published"
https://join-lemmy.org/docs/users/03-votes-and-ranking.html#votes-and-ranking
With that said... I have seen people comment in the past month that the active vs hot sorting might be a bit buggy still. Most (including myself) seem to prefer using the various top sorts.
Hope that helps!