Treadmill belts run on 2 parallel rollers. Belts can track from side to side on these rollers for several reasons including uneven floors, worn belts, uneven user impact and imperfections in the machining of the parts themselves. No matter what the reason, the fix is always the same: simply return the belt to the center position on the rollers. This is what is known as tracking the belt. Watch this video to see what to do.