What is the Flood

on the video game Halo, the flood is an ancient parasitic life form that lives inside the halo rings, it was herded and contained there by the forerunners. they can use any type of carbon based life form as their host, and some flood have become pure enough to stand on their own. the flood have no separate thoughts, only a collective entity called a gravemind. there are many of these scattered across the universe, and I think they are all inside halo rings. the halo rings are there to control the flood from spreading and infecting everything, and as a last resort, the ring can detonate with enough force to kill all life forms in a galaxy, as a way to control the flood.
i hope that was the answer you were looking for! ":-)

The Flood is not exclusive to Halo rings. They are also found on other Forerunner worlds that have yet to be found..