Well if you looking for some forerunner background here it is and is the lead in to "Halo 4"
The Forerunners the ancient race of enlightened beings that were the creators and builders of the Halo Array, the Arks, and the Shield Worlds, along with the artifacts that were found on the planets Reach. Later in their history, a Forerunner survey team came into contact with an extragalactic parasite known as the Flood on the planet G 617 g.
After this first contact, the Forerunner immediately recognized the great danger that the Flood posed to the entire galaxy and attempted to contain them. However, even they underestimated the potential of the Flood to learn and adapt, believing it to be a non-sentient but highly contagious disease. As such, the attempt to contain the Flood was a failure, as they tried methods more suited to disease control than warfare, trying to contain and quarantine the Flood rather than attack and destroy it outright. They did not start war until it was far too late.
Eventually, the Forerunners created the Halo Array, a weapon of last resort designed to starve the Flood to death by killing all sentient life with enough biomass to sustain them. And that's where the Halo rings come in on "Halo 1,2, and 3".