What does the EM heat mean on your thermostat for your central AC heating unit?

If you have a heat pump A/C unit then you have auxillary heat and emergency heat. The EM stands for emergency heat, which is using only your electric heat strips or gas heat, depending on your system. The auxillary heat uses your compressor inside of your outside unit. Say the house is 60 degrees inside and you set your stat to 70, the temperature difference is so great that if the heat pump alone cannot satisfy, then the auxillary heat would come on to assist the heat pump. Say you were to get a leak in either your condenser or evaporator coils, your heat pump would not work on the regular heat setting on the tstat. In this circumstance you would want to use just the emergency heat until a service tech can evaluate the system.