The timing belt tensioner pulley keeps the timing belt tight. It uses a spring or a hydraulic actuator to press against the smooth side of the belt. The internal pulley bearing must spin smoothly at engine speeds. If the pulley fails, the belt will slip, throwing off engine valve timing.
A seizing tensioner pulley will shred the timing belt quickly. On interference engines, this leads to pistons slamming into open valves. Always replace the pulley when you change the timing belt.