Look below the bottom of the last post in any thread. You will see a pulldown menu where you have the opportunity to 'rate' the thread you are reading. The star rating system is merely the average rating of all those who chose to rate that thread.
I think they should put that pulldown rating system at the top so that its more noticeable instead of at the bottom. Actually, they could do both (top and bottom) along with putting it next to the "Post a Reply" tab. Mark