This must be a recently introduced addon or I'm very oblivious.

http://danbooru.me/post/index?tags=order%3Arank

There's a new category under Posts next to Popular, which is Hot. I'm curious how this works, if someone could explain.

It was added about two months ago. It uses the ranking algorithm from Reddit to sort posts bases on their age and the number of favorites it's received.

See forum #59809.

I see. I did a search for 'Hot' and didn't get anything for it, so thanks.