Geothermal energy stems from the decay of naturally radioactive isotopes in the Earth's interior, which results in varying heat flow across the Earth's surface, depending on tectonic setting. In volcanic areas heat flow can locally be very high.
Geothermal electricity generation requires: heat (most effective in areas of high enthalpy); abundant heated water in an aquifer (or in artificial fractures in crystalline hot, dry rock); an impermeable cap-rock that prevents its escape.
Global heat flow, although renewable and present everywhere, is limited in its potential contribution to energy needs, being of the same order overall as global primary energy use in the early 21st century. More than two-thirds of it is through the ocean floor. Only small areas on land have sufficiently high heat flow to generate electricity. That will limit geothermal power to a small fraction of current electricity requirements.
Areas of low enthalpy can supply heat to buildings and other applications, thereby supplanting heating using fossil fuels.