Heating the passager cabin is done by piping in hot water from the engine's cooling system to the heater core under the dash where the controls for the heating and air conditioning are. The heater core looks like a miniature radiator similar to the one that is in the front of the the vehicle behind the grille to cool the engine down. There is a fan behind the heater core that blows air across it and is directed into ducts to blow out at the vents on the dash that you can position where you want the heated air to go.

 

Sometimes these cores leak just like a radiator and spill antifreeze on the floor of the vehicle or leak outside on the ground.

If this happens the core has to be replaced.

Putting stop leak in the cooling system will stop the leak temporally but is not a recommended fix as it can plug up other passages in the engine's cooling system.