From Halopedia, the Halo wiki
Hydrogen fluoride is a chemical compound with the formula HF. Together with hydrofluoric acid, it is the principal industrial source of fluorine and hence the precursor to many important compounds including pharmaceuticals and polymers (e.g. Teflon).
HF is widely used in the petrochemical industry and a component of many superacids. HF boils just below room temperature whereas the other hydrogen halides condense at much lower temperatures. As fluorine forms a hydrogen bond with hydrogen (The only other elements to do this are oxygen and nitrogen), it is not very soluble/acidic, even though it has a high difference in electronegativity.