Look at the parts of the word ... you've seen them both before:
"Geo ..." . . . has something to do with the Earth.
"... thermal" . . . has something to do with heat.
"Geothermal" . . . "Earth heat".
You know that the Earth's core is liquid metals, at high temperature. You may also know that when people go down into a deep mine, it's always warm down there.
Some of the heat from the Earth's core soaks up through the Earth, and there's actually a lot of it available without going too deep below the surface.
You may have read about "hot springs" ... pools of water that are always hot, because they extend down deep and get heated by the Earth's heat. And you may have seen pictures or videos of "geysers" ... deep holes in rock where there's water down at the bottom, which gets heated by the Earth's heat, and when it gets to boiling temperature, this stream of hot water and steam comes shooting up out of the hole. There are a lot of those geysers in Yellowstone National Park in Wyoming.
Well, if there's enough of this hot water close enough to the surface, then you can actually install pipes to bring it up to the surface, use the hot water to heat houses and buildings, and even use the steam to spin turbines and generate electric power ! Then you're using "Geothermal" energy . . . "Earth heat".
The country of Iceland is almost completely based on geothermal energy.
