In winter it means that hot water is not getting to the heater core. Could be low engine coolant level, or air in the cooling system. Some cars have a bleeder valve. Could be a thermostat that has failed and is stuck open. Could be a problem in the heater control system, and the wrong door inside is open or closed.
Could be a host of different causes. It would help to know make and model.
If your cooling system is very low on coolant fluid, your heater may not get enough hot liquid running through the heater core to work correctly.
If the plenum doors on the heating cooling system are malfunctioning it may not allow the air to blow by the heater core.
heat core bad
Turn the knob to hot then.,,,,,,
