Assuming you mean the thread rating, it's just a way that users can 'rate' a thread, with the average star rating appearing next to the thread in the forum.
When viewing a thread, at the top right next to the Black 'New Reply' button it says, Thread Rating: with 5 stars next to it, the current rating will appear in yellow, the rest grey. If you feel like using it, just click the number of stars you think it warrants (to clarify for example 4 stars you click once on the 4th star from the left). Or just go back to paying it no attention as I think most people do.
When viewing a thread, at the top right next to the Black 'New Reply' button it says, Thread Rating: with 5 stars next to it, the current rating will appear in yellow, the rest grey. If you feel like using it, just click the number of stars you think it warrants (to clarify for example 4 stars you click once on the 4th star from the left). Or just go back to paying it no attention as I think most people do.