A motorcycle thermostat is a component in the engine's cooling system that regulates the flow of coolant to maintain the engine at its optimal operating temperature. It is located between the engine and the radiator. The thermostat opens and closes based on the engine temperature, allowing coolant to flow through the radiator when the engine is hot and restricting it when the engine is cold. This helps prevent overheating, ensures efficient engine performance, and reduces emissions by allowing the engine to reach and maintain its ideal temperature quickly.