“Halo doesn't kill Flood. It kills their food. Humans, Covenant, whatever. We're all equally edible. The only way to stop the Flood is to starve them to death. And that's exactly what Halo is designed to do: wipe the galaxy clean of all sentient life.”
The 'Halo Array, Halos,' also known as the Sacred Rings by the Covenant, Fortress Worlds by their creators, Installations by the AI Monitors that run them, and the Great Wheels by prehistoric humans, are seven massive ring-shaped installations constructed by the Forerunners throughout the galaxy over one hundred thousand years ago as a last resort when combating the Flood. When nothing else could be done, they activated the rings which killed all sentient life forms within three radii of the galactic center. Not only are they weapons of last resort, but the Halo installations are also research facilities, mainly for the study of the Flood.