What is the function of a coolant thermostat in an engine's cooling system?

The coolant thermostat's main function is to regulate the engine's operating temperature by controlling the flow of coolant between the engine and the radiator. When the engine is cold, the thermostat remains closed to allow the engine to warm up quickly. Once the engine reaches the optimal temperature, the thermostat opens to let coolant flow to the radiator, preventing overheating. It does not display temperature or coolant levels; those are shown on gauges or indicators. Understanding this helps ensure the engine runs efficiently and avoids damage due to excessive heat or cold.