Basically, most power methods aim to move generators that create energy. I'm not going to go in depth as to how these generators work, but there needs to be a force that causes the generator to move. For example, with tidal turbines, the water moves the generator. With geothermal energy, you are using heat from the Earth's core to propel the generator (Usually, water is heated and the steam from that is utilized), thus creating the energy that we need. Geothermal energy is renewable (can't get used up) because the Earth will always have a heated core until the end of its existence. While fossil fuels can be used up, the Earth's heat and energy cannot.
