The Halo array is the last line of defense of an ancient race, known as the Forerunners, and was built about 100,000 years before the first Halo game takes place. The Forerunners engaged the The Flood, a aggressively violent alien race that could bind itself to sentient life and thus spread rapidly, in galactic war. The Forerunners battled the Flood as best they could but were unable to defeat them, so to save the rest of the galaxy from being overtaken, the Forerunners constructed numerous Halo arrays. The Halo rings were built to research and contain the Flood, but if the flood became too numerous, the Halo rings could all be activated and used to kill off all sentient life in the galaxy, depriving the Flood of a food source and thus eradicating them. The Halo array was activated once by the Forerunner known as Didact.
Many centuries later, when humanity and the Covenant became space faring races, the Covenant stumbled onto the first Halo ring. They worshiped the Forerunners as gods and saw Halo activation as a way of ascension. The Halo rings are integral to the conflict of the games, as the humans want to keep them deactivated and deal with the Flood any other way. The Halo custodians, sentient fore runner A.I's, want to fulfill their programming and wipe out the Flood by using the Halo Array. The Covenant want them activated to embark on a religious crusade they call the "great journey." Two Halos have currently been found, both orbiting gas giant planets. A third Halo ring was discovered, under construction, at the Ark. It was to act as a replacement for the one destroyed at the end of Halo: Combat Evolved. The Master Chief activated this ring, but due to it still being under construction, the force of the firing tore the construct apart, and did serious damage to the Ark. There were two attempts to activate them, one by the Master Chief and the other by Tartarus. the Arbiter and Miranda Keyes were successful in stopping Tartarus' attempt to activate Delta Halo, but the Master Chief successfully activated the spare Halo above the Ark, however, its pulse only killed the Flood because it was out of range of any inhabited world.
The Halos are cared for by advanced AI-controlled robots know as monitors. Two have been found so far, Installation 04's 343 Guilty Spark and Installation 05's 2401 Penitent Tangent. They are in charge of all the other automatons on Halo, such as constructors, sentinels and enforcers. The custodian's primary role is to keep its Halo ready for firing and to contain any Flood outbreaks.
Since the Halos contain their own ecosystem they also support wildlife. Wild animals are never featured prominently in the Halo games, however; they are simply represented as sound effects and creatures flying in the distance. In an early E3 video, a dinosaur-like creature and an animal named the blind wolf were shown. There was also mention that a creature called the doberman gator would be used as a vehicle of sorts. These were removed due to the designers' choice that the absence of all other animal life would make the unexpected appearance of non-Covenant aliens in the form of the Flood more notable and scary. In Halo 2 and 3, birds in the sky can be shot and killed, exploding in burst of blood and feathers.