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 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 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. 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 briefly refered to in Ghosts Of Onyx, one of the Sentinals does speak and refers to one of the Spartan-III'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.

Also, after the Spartan-III responds, the Sentinel reclassifies him as an "aboriginal sub-species." One could argue that the Spartan-III's are a lesser form, or sub-species, of the Spartan-IIs.

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 supported by the scene 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.

Humanity could be a re-evolution of the Forerunner in case the Ark and Shield worlds fail to save them from the effects of the Halos, a Plan B of sorts. Examples are: The Master Chief and the other Spartan-IIs describe Forerunner technology and language as "familiar," and humans have not been invited to join the Covenant because the Prophets are afraid they would replaced as leaders.

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 arbitrary standard (the Forerunners?) than a merely organic mind. As all UNSC personnel have cybernetic implants of varying qualities, this may mean that a civilian human would not be regarded as a Reclaimer. This is, 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?"(Please note that at this point in the game the Chief/Player has just killed an army of Flood to retrieve the index and that 343 Guilty Spark could be asking why the Chief would not be willing to kill, having already done so) 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 the novel Halo: Ghosts of Onyx the Spartan IIs say that the Forerunner technology seems familair. 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. Another speculation is that the implants that the original Spartan II's were given, (since being in a very narrowed, specific group)were only gene-specific to the surviving Spartans. Evidence that supports this includes when the Sentinel attempts to speak to Spartan G099,Ash, being a Spartan III,(the genetic enhancers and implants the Spartan III's were given were not Gene-specific, as the Spartan II's were) and identifies him as a "sub-species reclaimer", and also the familiarity the Spartan II's felt with the forerunner   texts and symbols, as the same feeling is never stated or hinted towards the Spartan III's.

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. It could also suggest that as a Reclaimer, the Chief would be refered to as a Forerunner so therefore has done it generally as a race.

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. Or it could be that the Chief already put the index in and tried to activate the ring. However Cortana took the index from inside the mainframe and did not allow Halo to be activated. So when 343 Guilty Spark says "Why do you hesitate to do what you've already done?" it is refering to the Chief already trying to activate Halo.