Geothermal energy is available in the form of differences between the temperatures of the earth's surface and of depths beneath its surface at a given location. Such temperature differences can be exploited for heating and cooling, for example, by installing a ground-source heat pump.
The DOE's Energy Savers site says:
Geothermal Heat Pumps
Geothermal heat pumps (sometimes referred to as GeoExchange, earth-coupled, ground-source, or water-source heat pumps) have been in use since the late 1940s. Geothermal heat pumps (GHPs) use the constant temperature of the earth as the exchange medium instead of the outside air temperature. This allows the system to reach fairly high efficiencies (300%-600%) on the coldest of winter nights, compared to 175%-250% for air-source heat pumps on cool days.
I had always thought that a system's efficiency is less than 100%. I think they mean energy efficiency ratio or coefficient of performance, which is the ratio of the transferred heat energy to the input electric energy.