The stability system in your 2003 GMC Yukon Denali is part of the anit-lock brake system and helps keep the vehicle under control during driving conditions when you may begin to slide out of control when turning. When the service stability lights comes on fault code(s) are stored in the control module and the system is disabled. These code(s) may be retrieved using a scan tool. Reviewing and diagnosing the fault codes and then repairing any issues found should bring the stability system back to proper working order and the warning light will go off. The most common problems with the stability system are with the steering wheel and yaw rate sensors.