The coolant reservoir, also known as the expansion tank or overflow tank, is a container that holds excess coolant from the motorcycle's cooling system. It is designed to accommodate the expansion of coolant as it heats up and to ensure there is always an adequate supply of coolant available for the engine. Positioned near the radiator or cooling system, the reservoir helps to maintain proper coolant levels and prevents overheating by allowing coolant to return to the system as it cools down.