Reclaimer

A "Reclaimer" is an individual who, whether by chance or by precedent, has been placed in charge of retrieving an item known as the Index, returning it to, and subsequently placing it into a control panel within a control center on a Halo structure to activate a massive superweapon. The word is first used as a title bestowed upon a Staff Sergeant Marvin Mobuto in Halo: The Flood and Master Chief in Halo: Combat Evolved by an artificial intelligence construct, designated 343 Guilty Spark by its creators, the Forerunners.

History
The only known role of a Reclaimer is to recover the "Index" and return it to the Halo control center. Doing this would activate Halo's ability to destroy all sentient life within twenty-five thousand light years, and would also activate other Halos, causing the entire galaxy to be purged of sentient life. This would be done to prevent the spread of The Flood, which can only be accomplished by destroying all sentient life with a biomass sufficient enough to sustain them. This would then destroy the Flood via starvation.

343 Guilty Spark assumed, for unknown reasons, that the human soldier Master Chief was a Reclaimer. However, Commander Keyes and Sergeant Johnson were also called Reclaimers by 343 Guilty Spark. Penitent Tangent also refers to Master Chief as a Reclaimer, implying that all Monitors recognize him as such. In addition, in Halo: The Flood the Master Chief encounters the corpse of Sergeant Marvin Mobuto in the Library, whom Spark also refers to as a Reclaimer.

It is very likely that all humans are Reclaimers, as 343 Guilty Spark confirms that humans are Forerunners. It would also explain why the Covenant cannot activate the Halo rings; the Forerunners probably weapon that could only be fired by the Forerunner race as a safety measure, so future races could not activate the rings accidently.