Geothermal Energy has been around for as long as the Earth has existed. "Geo" means earth, and "thermal" means heat. So, geothermal means earth-heat.
Hot water or steam from below ground can also be used to make electricity in a geothermal power plant.
Here's a cut-away showing the inside of the power plant. The hot water flows into turbine and out of the turbine. The turn turns the generator, and the electricity goes out to the transformer and then to the huge transmission wires that link the power plants to our homes, school and businesses.