Geothermal energy is heat generated inside dense and rocky cosmic bodies. It can be generated by the immense pressure in the core of planets and created by nuclear processes and is constantly available with virtually no limit in time.
On Earth there are a few installations to use geothermal energy for heating and for production of electricity. On Mars there might be usable geothermal energy, as well. It is unknown how deep a Mars colony would need to drill for practical Geothermal energy. There are signs of geothermal energy, however.
Signs and theories
Mars Global Surveyor has found relatively fresh signs of flowing water in the Cerberus region near the equator of Mars. This probably happened 10 million years ago, which is geologically an extreme short time. Hopefully, the heat is still there in certain spots. 
- Are there scientific theories about the inner temperature on Mars?
- Are there spots with usable geothermal energy? Additional explorations on Mars are needed to get exact figures.