A car radiator disperses heat away from the main engine block. During the combustion process, a car engine burns fuel (whether petrol or diesel), and generates a considerable amount of heat. What's more, a car engine has many moving parts; this movement creates friction, which in turn adds to the heat.
Without a radiator to regulate these extreme temperatures, your car would constantly overheat and seize. When the engine reaches a certain temperature, the thermostat opens to allow the flow of coolant and water around the engine and back to the radiator. This causes it to lose heat to the cooler surrounding air. The front grill of the car allows outside air to blow over the radiator. If the radiator gets too hot, either due to the weather or when the car is stationary, a fan behind the radiator blows air over it.