What do a heater cote do?

A heater core is built into the firewall behind the glovebox. It lets hot engine coolant run through it and the blower blows over it allowing heat to come into the car through the vent system. Before you jump to a plugged or bad heater core, they are sometimes quite expensive to replace, check the coolant level in the radiator and overflow bucket. If they are low fill them to the proper levels, then look for a leak. If they are not low it could be a bad thermostat.