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.
Active is either brand new threads like this one (at the time of posting this comment) or older threads with lots of actual activity in terms of comments and upvotes.
Hot is generally more consistent in showing threads from within the last 24 hours, including threads with smaller amounts of activity.
There is an option in the Lemmy website that it is called show read posts, when you disable it all your read posts are hidden, and this setting does the same behavior for the Lemmy apps, I think only Jerboa and Summit have Lemmy web preferences, lately I have been interacting with this from Summit.