From Halopedia, the Halo wiki
Helium, symbolised as He, is a colourless, odourless, tasteless, non-toxic, inert monatomic chemical element that heads the noble gas series in the periodic table and whose atomic number is 2. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Extreme conditions are also needed to create the small handful of helium compounds, which are all unstable at standard temperature and pressure. In its most common form, helium-4, it has two neutrons in its nucleus, while a second, rarer, stable isotope called helium-3 contains just one neutron.
Helium is the second most abundant and second lightest element in the known universe, and is one of the elements believed to have been created in the Big Bang. In the modern universe almost all new helium is created as a result of the nuclear fusion of hydrogen in stars. Helium is present on some planets, such as Te and Balaho. On Earth, helium is rare, and almost all of that which exists was created by the radioactive decay of much heavier elements (alpha particles are helium nuclei).
Both the UNSC and the Covenant use the helium isotope helium-3 as fusion fuel. In human warships' fusion drives, He-3 is fused with deuterium to provide both power and direct propulsion for the ship. The Covenant also use helium-3 as starship fuel, extracting it using large machines known as Beacons. Due to its lightness, helium also fills a good number of a Huragoks' dorsal sacs.