Geothermal energy is heat from the earth
The internal structure of the earth represents a vast store of thermal energy. The temperature at the base of the crust is 1000˚C which means wherever you are the rocks get hotter with depth. Along tectonic plate boundaries, and in volcanic regions, high temperatures can be found quite close to the surface and countries like Iceland and New Zealand have been exploiting geothermal energy for many years by drilling shallow wells.
Where the temperatures are high enough, geothermal energy can be used to generate electricity.
Where they are lower, geothermal fluids can be used to provide direct renewable heating.