An MMRTG (for Multi-mission Radioisotope Thermoelectric Generator) is a standardized design for space-probe power units. The design generates 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's orbit.
There have been two production MMRTGs, both in Mars rovers: one in Curiosity, and a second launched in 2020 in Perseverance, but due to a shortage of suitable fuel, only two more are possible.