A coolant reservoir is a storage tank that holds a supply of coolant, which is used to regulate and maintain the temperature of an engine or other system. The coolant absorbs heat from the components it circulates through, such as the engine block, cylinder head, and radiator, and then dissipates this heat into the air or surrounding environment. By doing so, it helps prevent overheating and ensures that the system operates efficiently within its designed temperature range. The coolant reservoir is an essential component of a cooling system because it provides a convenient means to view and maintain the proper level of coolant in the system, which is necessary for optimal performance.