In the Halo universe, the ancient species that built the Halo ring installations is the Forerunners.
The Forerunners were a technologically advanced civilization that governed much of the galaxy before disappearing roughly 100,000 years before the main games. Their surviving technology includes the Halo installations, shield worlds, portals, artificial intelligences and many weapons.
The rings were created during the Forerunners’ war against the Flood. Rather than functioning as ordinary weapons, they were designed to kill sentient life across a huge area, removing the Flood’s available food. The Forerunner plan also preserved selected species so life could later be reseeded.
The Forerunners are distinct from the Precursors, an even older civilization connected to the origins of both the Forerunners and humanity. The Covenant initially worships Forerunner technology without understanding its true purpose.