Belts stretch over time as they run. If a belt gets loose, it will slip on the pulleys, producing a loud squeal and stopping the A/C compressor from spinning. The A/C belt tensioner resolves this. On older cars, this is a manual slide pulley that you lock down with a bolt. Modern engines use automatic tensioners. These feature an internal spring that pushes a pulley against the belt, maintaining constant tension even as the belt stretches.
Tensioner bearings wear out over time. When the bearing fails, it will make a chirping or whirring noise that increases with engine speed. The spring inside automatic tensioners can also weaken, letting the belt flap around. In severe cases, the pulley tilts, causing the belt to walk off the track and snap.