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 and 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 Master Chief by an artificial intelligence construct, designated 343 Guilty Spark by its creators, the Forerunner.

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 the Flood, this would then destroy the Flood via starvation.

343 Guilty Spark assumed, for unknown reasons, that the Earth soldier Master Chief was a Reclaimer. However, Commander Keyes and Sergeant Johnson were also called Reclaimers by 343 Guilty Spark. 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.

Other Interpretations
''The above is one possible interpretation. It is not stated explicitly in either gameplay or literature that Reclaimer is even a title. Neither The Master Chief nor Commander Keyes are ever referred to as the Reclaimer. The Monitor always uses the terms Reclaimer, a Reclaimer, or this Reclaimer.''

Reclaimers are literally Reclaimers
Although only breifly reffered to in Ghosts Of Onyx one of the sentinals does speak and refers to one of the Spartan 3's as a reclaimer this might mean that all forerunners involved in the halo project were reclaimers, or the people that hid in the shield were meant to come back and "reclaim" all of the forerunners technology after the flood was destroyed.

Reclaimer is another word for human
Another interpretation is that a Reclaimer is the Forerunner terminology for a human. From this it is possible to assume that the Forerunners had knowledge of humans and delegated to them the task of reclaiming Indices and activating the rings, among many other speculative duties. This theory is confirmed when an Onyx Sentinel chasing Spartan-III G099 attempted to communicate with him, calling him a Reclaimer.

Forerunners are human
Another interpretation is that the Forerunners were human, or at least, very closely related, perhaps being direct ancestors or future form of humanity. There is evidence to prove this, such as 343 Guilty Spark's statements in the first game, and the fact that the Covenant have not asked humanity to join them. This could infer that as the humans learn more about the Forerunners, they are in fact reclaiming their lost technology and or position in the universe.

In Conversations from the Universe, the Monitor has a list of slang used by human forces to describe Covenant troops entitled "Cross reference Reclaimer terminology/nomenclature/slang for hegemonizing swarm "Covenant" against self-descriptors." As the Master Chief is rarely seen to speak throughout the games, and never in combat, this seems to suggest that all humans may be Reclaimers.

Enhanced means Reclaimer
Another interpretation is that is not the species, but their cybernetic implants that mark the Master Chief and Commander Keyes as Reclaimers. Gravemind also referred to the Master Chief's "machine and nerve" mind as "concluded", which may mean it is closer to some arbitary standard (the Forerunners?) than a merely organic mind. As all UNSC personnel have cybernetic implants of verying qualities, this may mean that a civilian human would not be regarded as a Reclaimer. This, as is much of the Halo universe, all highly speculative.

(Given the situation, machine and nerve could simply be a reference to his cybernetical enhancements and has his mind 'concluded' in ending this war, while the Arbiter was 'merely flesh and faith and is the more deluded' in that he had no modifications and was blind in his faith towards the covenent beliefs)

Forerunners evolved into humans

 * Another possibility is that the Forerunners interbred with humans passing on their genes. Master Chief, Johnson, Mobuto, and Commander Keyes would then be genetic descendants of the Forerunners. This is also supported by 343 Guilty Spark's reactions to Master Chief in Halo: Combat Evolved, where he assumes that Master Chief understands everything that has happened on Halo in the book The Flood and he also states that the Master Chief is the one that activated Halo the first time in the level Two Betrayals (Levels). "Why do you hesitate to do what you've already done?" This could mean that Master Chief's ancestor was the Forerunner that activated Halo. Also supporting this theory is that in the book The Fall Of Reach, Dr. Halsey chose candidates for the Spartan program based on certain genes they carried. It is probable that those genes are those of the Forerunners. In addition to this, in the novel Halo: Ghosts of Onyx an onyx sentinel refers to Spartan-III G099 as an '"aboriginal subspecies"'. At first, the Sentinel labels him a Reclaimer and attempts to communicate.  When he fails its 'test', it attacks. But as always, this is highly speculative.

Time Travel
Working with the same quote, from the Two Betrayals (Levels). "Why do you hesitate to do what you've already done?" this line perhaps suggests time travel is involved, and that Master Chief in some future game travels back in time to activate Halo, and the story comes full circle.

Insanity
However, the above can be contradicted with the fact that 343 Guilty Spark is arguably insane. When he made the quote "Why do you hesitate to do what you've already done?" and recited a previous conversation he had with an unnamed individual, Spark may very well have been referring to a different Reclaimer from when Halo was first activated and, due to his insanity, conceived the Master Chief as the original Reclaimer.