A radiator helps to eliminate excess heat from the engine. It is part of the engine's cooling system, which also includes a liquid coolant, hoses to circulate the coolant, a fan, and a thermostat that monitors the coolant temperature. The coolant travels through the hoses from the radiator, through the engine to absorb the excess engine heat, and back to the radiator. Once it returns to the radiator, thin metal fins release the heat from the coolant to the outside air as the hot liquid passes through it. Cool air flows into the radiator through the car's grille to aid in this process, and when the vehicle isn't moving, such as when you're idling in traffic, the system's fan will blow air to help reduce the heated coolant's temperature and blow the hot air out of the car.

Source: J.D. Power