A serpentine belt is made of rubber molded over a core of stranded natural/synthetic fiber cords. Over time, the rubber will dry rot and crack. In some cases the belt/cords will stretch and if the belt doesn't break first, the slop from the stretching could cause your vehicle accessory systems (electrical, power steering, water pump, AC) to not function properly. You should inspect your belt at every oil change or have someone check it for you so that you won't get stranded from a broken, rotten, old serpentine belt.