Mantle

"The Mantle. You still hold to that [belief] after all that has happened? After this thing has consumed a million worlds?
Can't you see? Belief in the Mantle sealed our doom! Weakened our [protectorates], bred dependence and sloth. Our [so-called Guardianship] has stripped those we would keep safe of any capacity for self-defense!
Were we such noble [Guardians] when we drew our line and abandoned billions to the parasite?"

— Librarian

The Mantle was a Forerunner philosophy referred to by Terminals found on The Ark. While no precise definition of it exists, it is heavily implied that it was the source of the Forerunners' "authority", and was the reason why they fought the Flood - to protect their "charges" from the xenocidal parasite. Forerunner mythology held that the Mantle was passed down to them by an even earlier and more advanced race, the Precursors, though to what extent this was true, or even whether it was a common belief or a rarity, remains unknown. Regardless of its origin, the Forerunners seem to have handed their Mantle down to humanity in their turn before activating the Halo Array, destroying themselves and crippling the Flood but saving what was left of the galaxy's sentient life, and may play a significant part in humanity's Reclaimer status.

Many Forerunners doubted the purpose of the Mantle, dismissing it as superstition and fairy tales. The Librarian, in particular, doubted that it was useful or real, stating that she believed it responsible for the downfall of the Forerunners at the hands of the Flood. The rampant AI Mendicant Bias believed that the Forerunners used the Mantle to obstruct any change, even that which might eventually be beneficial, and allied with the Flood against them. Rather than allowing every species to rise on their own merits, the Forerunner's, following their Mantle, gave them no need to do so, serving as protectors against any threats might have forced technological and cultural progression. An unforeseen side-effect was that only the Forerunners were able to hold their own against the Flood, while their charges were unable to defend themselves, unintentionally leaving the galaxy vulnerable to Flood infection.

Simply put, the Mantle was the Forerunner's belief that they had to preserve biological diversity in the galaxy, which would provide one reason for their mass self-sacrifice by creating and firing the Halo Array in an effort to at least spare humanity and other sentient life. Thus, they were essentially the "Guardians of Life in the Galaxy".