The Mantle [1] is mentioned in the Terminals found on the Ark and the second Installation 04. While no exact definition of it exists, it is heavily implied that it is the source of their "authority," and is the reason which they fight the Flood - to protect their "charges" from the Parasite. It has been implied, albeit in a derogative manner, that the Mantle was passed down to them by another species, speculated to be the even more mysterious Precursors.
Trivia
- The Mantle was, at one time, thought to be a Forerunner government.