Helium 3 is a light isotope of helium, having 2 protons and only 1 neutron. It is the only stable isotope of any element with more protons than neutrons.
So what makes it so special?
All nuclear power plants on Earth use nuclear fission reactors, in which uranium nuclei are split, to release energy. Unfortunately, radioactive waster is also released through this process and needs to be safely stored. To stop the unwanted radioactive by product, scientists have been focusing on mastering nuclear fusion, which does not produce radioactivity and nuclear waste. Nuclear fusion reactors use hydrogen isotopes tritium and deuterium as fuel, with being energy released when their nuclei fuse to create helium and a neutron. However, the neutrons released here lead to a significant amount of energy being lost. A potential solution to this problem would be using helium 3 and deuterium as the fuels, which would in turn create helium and a proton. This would allow for an optimum situation with virtually no waste and no radiation.
HOWEVER... there is still one more big problem!
Helium 3 is extremely rare to source on Earth - there is a net supply of around 15 kg a year. The sun emits helium 3 within its solar winds but unfortunately the Earth's atmosphere prevents any actually arriving.
But, luckily for us, our closest neighbouring space object (The moon!) does not have an atmosphere and so contains an abundance of helium 3. It is estimated that there is around 1,000,000 metric tonnes. This is enough to power human energy needs for over 10, 000 years!
Extraterrestrial real estate is still an unrecognised standing and so owning anything that is not on Earth still doesn't hold any official weight. But imagine if you could buy a portion of the moon and set up a factory to mine helium 3 and harness this future energy solution... that would be a company worth starting!