An MMRTG (for Multi-mission Radioisotope Thermoelectric Generator) is a standardized design for space-probe power units that generate power using the Thermoelectric Effect (a means of inducing a voltage from a difference in heat) from radioactive heating in its Radioisotope Heat Unit (RHU). The radioactive fuel in the case of an MMRTG is Plutonium-238 Dioxide. ("Multi-mission" indicates multiple missions will use the same design.)
MMRTGs (and predecessors and other devices based on the same principles) are used for missions requiring more electric power than practically capturable from the Sun, typically for planet exploration beyond Earth.
The only production MMRTG was used by the Mars rover, Curiosity, but a shortage of suitable fuel will limit future uses: current plans are to build only three more, including one for a planned Mars rover.