You're right, the description on the site is not very helpful, I'll add that.

TL;DR It's just a visualization layer above the HN frontpage. Hot means a post has either 'a lot of comments' or 'a lot of points' (configurable with the filter dropdown). It's not much but I was wondering whether anyone would be interested in having such a way of looking at HN. If there's someone who found that useful I'd probably develop it further