I found this in the vBulleting FAQ:
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How do I rate a thread?
You may find a small menu on bottom, right-hand side of thread pages which allows you to 'rate this thread' with a number between 1-5.
Casting a vote for threads you view is entirely optional, but if you think that the thread is superb, you might rate it as a 5-star thread, or if you think that it's unspeakably dismal, you might choose to rate the thread with a single star.
Once enough votes have been cast for the thread rating, you will see a set of stars appear next to the title of the thread in the thread listings. These stars reflect the average vote cast, and allow people to quickly see which threads are worth reading if you are on a very busy board with a lot of threads.
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Rating requires several people to rate a thread before any rating appears in the list of threads in the top level of the forum or on searches. If we all pitch in and flag the best threads (and the worst ones) it will help everybody here to recognize what threads are the best reads.
It's especially useful when people do searches for old threads--they often get a big list and have to waste a lot of time clicking each one to see if it any good. This way, if a thread is good (or bad) they can see it in the search results.
What say you guys? Can we get together and do this for the benefit of all?