What exactly ARE the Halos in the video game series Halo?

The halos, or scientifically, the installations were a way of getting rid of the flood, a parasitic life-form that infects any living thing it consumes. And you're probably wondering how they got rid of the flood. Well, the halo rings would be fired and destroy everything in range, starving the flood to death. A race of alien species known as the Forerunners built these rings. There were very advanced in technology, but the flood caught them off-guard. The Forerunners finally fired the rings, and suddenly vanished from existence. Hundreds of thousands years later, a Slip-space starship wandered into an unknown region trying to lure an alliance of alien species, the Covenant, away from Earth, and the Master Chief was forced to find the secrets of the Forerunners' extinction and the Halo rings.