Heat pumps are combined heating and cooling systems that push/pull heat from a cooler place to a warmer space
For example, from inside to outside during summer to cool interior spaces, and from outside to inside during winter to help heat interior spaces. This is why the back of a refrigerator gets hot: it is pulling the heat from inside the box to the outside coils to lower the inside temperature. The same idea can be used to heat and cool buildings. Heat pumps can be geothermal or ground-source, air-source, or water-source depending on the climate at installation and their designed use.