What are ratings?
The forums allow you to rate threads between 1 star (terrible) and 5 stars (excellent). Once enough votes are cast for a thread, stars will appear next to its name in the listings. These show the average vote, and can be an easy way to see which threads are worth reading if you are on a busy forum.
On the forum viewing page you can also arrange threads by rating, with either the highest or lowest at the top.
It therefore makes sense to rate threads because it helps all users. To do this, click on the 'rate thread' link at the top of the thread viewing page. Choose the number of stars you feel best represents the quality of the thread. You may or may not be able to change your choice of rating at a later date.