When 343 Guilty Spark and Master Chief met, 343 called Master Chief a "Reclaimer". The word is also used several times in the books. What does it mean? Did the Forerunners leave the artifacts all over the galaxy for the benefit of the Humans?
Master Chief was called "Reclaimer" by both 343 Guilty Spark as well as 2401 Penitent Tangent. 343 Guilty Spark did not call Master Chief "Reclaimer" at the time of their first contact. Here are the lines (I've just played Halo: Combat Evolved to get that):
"Greetings. I am the Monitor of Installation 04. I am 343 Guilty Spark." [pause]
"Someone has released the Flood. My function is to prevent it from leaving this installation. But I require your assistance. Come. This way."
Then, he teleported Master Chief inside the Library.
A Reclaimer (Latin: Homo Sapiens Augeous) is an individual who has been placed in charge of activating a Halo super-weapon. The individual must journey to retrieve the Halo's activation key (the Index) from the ring's Library and then use it to activate the Halo Array. It has also been demonstrated that it is the role of a Reclaimer to activate Forerunner technology on installations such as the Ark or the Shield Worlds.
This is something that has been explored in lore quite often already actually.
Both Halo: Contact Harvest, shows that all humans are considered Reclaimers and are marked as such by Covenant technology (which was basically Forerunner technology that they either reverse-engineered, or used as is in their ships), as well as the terminals from Halo 3, where the Librarian considers humanity to be "special" and builds a portal to the Ark on Earth so she could send them back for preservation.
It is also an element that was explored in the recent (and still ongoing as of this answer) Forerunner Trilogy. Halo: Cryptum (book 1) covers most of the Forerunner history before the Forerunner-Flood war and details some elements about how Humanity was the only force that could combat the Forerunners back in their day (until the flood decimated their worlds). Forerunners then devolved those humans but after the flood resurgence, needed the same devolved humans to aid them, culminating in one of them becoming the basis of Guilty Spark.
While the Didact, the military command of the Forerunners during both the Forerunner-Human war and the Forerunner-Flood war, looked down on humans, he grudgingly respected their tenacity and adaptability to their environment. Combined with the imminent destruction of his race via the Halo Array, it isn't surprising he would choose humanity to inherit, reclaim, the legacy of the Forerunners, especially since his wife, The Librarian, respected them so much.
This reclaimation by humanity is also tied to Forerunner's religious belief in a "Mantle" of guardianship of the galaxy. This Mantle was the idea that the current dominant galactic superpower was the guardian of the weaker species. Naturally this played out a bit too idealistically and was a reason that lead to the Forerunner-Human war, but that's a politically philosophical argument that's also explored. But the Forerunners believed this Mantle had been passed down to them by the Precursors and so Didact wished to preserve this idea and passed the Mantle onto Humanity, hoping that they would not make the same mistakes as the Forerunners such as allowing themselves to become too homogenized in terms of culture and ideas (an idea explored also in Mass Effect 3 via the 'From Ashes' DLC and the rest of Mass Effect 3) which allowed the Flood to quickly expand without too much damage early on in the Forerunner-Flood War.
Currently this is unknown, but I suspect the upcoming Halo trilogy (starting with Halo 4) will address this, as Microsoft recently revealed the new trilogy will be called the Reclaimer Trilogy.