A heat pump system is similar to a conventional home A/C system. But it also has the ability to heat your home in addition to cooling it. In the summer it cools the home, and in the winter it reverses the same process and heats it. These are very popular systems in areas where the winters are very mild, like Dallas/Fort Worth.
Heat always moves from hot to cold. The greater the temperature difference between the two areas is, the faster the heat moves from hot to cold.
In order to understand how this applies to the heat pump, we must first explore how an A/C system works at a very basic level. There is an indoor coil and an outdoor coil. The indoor coil temperature is about 40 degrees. So when you blow the house temperature air through it, it absorbs heat from the house temperature air, because heat moves from hot to cold. The absorbed heat is then sent to the outside unit to be disposed of.
The heat pump works identically to an A/C unit in the cooling mode. But in the heating mode it simply reverses the process. The outside unit absorbs the heat from outdoors and sends it to the inside coils to be disposed of. So we utilize that heat to heat our home.