The heater core is part of a car's cooling system.

It looks and acts like a smaller version of a radiator, circulating coolant through the little tubes and radiating heat into the cabin.

The simplest systems moderate the temperature by use of a blend door, mixing outside air with air from the heater core. 

It increases the temperature inside the vehicle by using the hot coolant, which is heated from the engine, and removes the frost on the car window and maintains the proper temperature by functioning the air-conditioning together.