That light tells you that it is PREVENTING clipping. It works like a limiter, and won't allow anymore sound to enter the speaker area. Once it max's out to a level it can handle, the limiter will not allow harmfull, distortion to damage your speakers. (at least, that's the plan !)
Simply put ... there is no more power available for you. Maybe you can make a few adjustments and maximize your effective volume, but chances are, you are at the max now. You probably need a more powerful amp if that light stays on all the time.