Each Voyager spacecraft is powered by three radioisotope thermoelectric generators (RTGs).
The Voyagers travel too far from the Sun for solar panels to provide practical power. Their MHW-RTGs use heat released by the radioactive decay of plutonium-238 and convert part of that heat into electricity through thermoelectric couples. The three generators are mounted together on a boom away from the spacecraft's sensitive instruments.
At launch, the three RTGs supplied about 470 watts of electrical power at 30 volts. Their output gradually declines because plutonium-238 has an 87.7-year half-life and the thermoelectric components also degrade. This is why mission controllers have progressively switched off instruments and heaters while keeping the spacecraft communicating.
An RTG is not a nuclear reactor and does not propel the spacecraft. It is a long-lived power source, producing electricity and waste heat for instruments, computers, and communications in the cold outer Solar System.