The Forerunners are a long-lost species which are first cryptically mentioned in Halo: Combat Evolved. Little is revealed about the Forerunners in the games, but for unknown reasons they consider humanity to be "special"; humanity is thus the only race which can reclaim the keys which activate the Halo network. According to the official backstory to the series, the Forerunners came to power after the extinction of an even more ancient race, known to them as the Precursors,[27] and formed a galaxy wide empire more than 3 billion years before the events of the main Halo story arc.[28] The Forerunners took it upon themselves to protect all life in the galaxy, but were threatened by the parasitic Flood. The Forerunners took action, but the parasite was unstoppable. A group of Forerunners conceived a plan to stop the Flood once and for all, building an installation known as the Ark that created seven ring-shaped megastructures called "Halos". The Halo Array, when activated, would destroy all sentient life within range—depriving the Flood of its food. After waiting as long as they could, and travelling the galaxy to collect species from planets, which would later be used to "re-seed" the galaxy, the Forerunners activated the Array and vanished.[29] Although the parasite's spread across the galaxy was halted, the Forerunners paid a terrible price as they, and all sentient life capable of sustaining the Flood in the galaxy, were annihilated. The Covenant worship the Forerunners as deities and relentlessly search for Forerunner relics. The reason for the Forerunner's disappearance is revealed in Halo: Combat Evolved, when the artificial intelligence Cortana enters one of the Halo's computer networks and learns the true purpose of the Halos.
-http://en.wikipedia....Halo#Forerunner
For more information about Forerunners, go to this website: http://halo.wikia.com/wiki/Forerunner