If you have a heat pump, your thermostat likely has a heat strip setting. This tells the thermostat to use electric resistance heating coils to supplement or replace the heat pump's normal operation. The heat strips come in different sizes, and their wattage will determine how much heat they can generate. Most homes with heat pumps will have a 3 kW or 5 kW strip.
The main reason to use the heat strip is for backup heating on very cold days. The temperature outside may be so low that the heat pump can't extract enough warmth from the air to meet your home's needs. In this case, the electric coils will provide most of the heating, and they'll run until the temperature outside rises and the heat pump can take over again.
The other common reason to use the heat strip is if you have a zoned system and one zone is particularly cold. In this case, you might only need to turn on the coils in that one zone rather than heating your entire home with them.
Heat strips are relatively inexpensive to operate, but they do tend to wear out faster than other parts of your HVAC system. That's why it's important to keep an eye on them and have them serviced regularly by a professional HVAC technician.