You will see a 5 blank stars on thread pages which allows you to rate each individual post within any thread' with a number between 1-5. Simply scroll your mouse over the stars in the posting and click on the number of stars based on your rating.

Voting for posts you view is entirely optional, but if you think that the post is amazing, you might give it a 5-star, or if you think that it's horrible, you might choose to rate it with a single star.

Once enough votes have been cast for the post rating, you may see a set of stars appear with the title of the post. These stars reflect the average vote cast, and can allow you to quickly see which posts/threads are worth reading.