Original: Each probe gets its electrical power from three radioisotope thermoelectric generators (RTGs), which are expected to continue to generate enough electric power to let the probes keep communicating with Earth at least until the year 2025.
Simplified: Both Voyager probes are powered by three radioisotope thermoelectric generators, which have far outlasted their originally intended lifespan, and are now expected to continue to generate enough power to keep communicating with Earth until at least around the year 2020.