This plugin is no longer maintained.
Hot topics are the ones that get the comments. This plugin lists the posts that got the most recent comments, without duplicating a post that got more than one comment recently. You set a maximum number of posts to list and a "hotness" factor. The "hotter" you set it the less posts you're likely to see in your hot topics list (due to multiple comments on a post). Kinda groovy if you're into this sort of thing :)