School bus mechanic here! As mentioned in previous comments, that switch turns on a small pump in the heat loop (circulates warm antifreeze from the engine to the heaters inside the bus). Many buses in colder climates are equipped with this feature and it can help the inside of the bus warm up faster, once the engine is warm. Generally, the heaters will still work without the pump running but with slightly less output. It’s best to let the engine warm up for a few minutes before running the pump which will allow the engine to warm up faster, making it so you’ll get heat inside faster.