Helium-3 is a light non-radioactive isotope of Helium. It is used as fuel for nuclear fusion power plants. It is very limited on planets like Earth, however there are large amounts located on places like Earth's moon and other select areas. In the Leijiverse, it is used for powering of the most high-tech and sophisticated vessels such as spaceships and Metanoids.
Helium-3 in a large amount on the moon allowing Earth residents to mine the material and complete the Nuclear Fusion process with less risks of fallout and radiation. Other sentient species had also found Helium-3 in certain areas allowing them power sources similar to what Earth has accomplished.