The thermostat is a valve that controls the flow of coolant in the cooling system of the vehicle.
When you are concerned about the routine maintenance of your car, it is natural to get to a point where you will ask yourself, what is a thermostat? And more importantly, what does it being stuck closed, or even stuck open for that matter, mean to me and my car? A thermostat regulates the flow of coolant flowing to the engine.
When you get into your vehicle, you simply expect the air conditioner or the heater to work when you need them.