Answer:
Halo is what they call the giant ringlike structure that is alot like Earth but has the land and ground on the inner side of the ring. It is both a habitarium and a weapon, as well as a containment facility to study the Flood. The Flood is the entire reason the Halos are Built, as they are designed as a last resort weapon, to prevent the spread of the flood by killing all food sources. Halo's creators, the Forerunners have died upon it's activation.