By the power of truth, I, while living, have conquered the universe.
The Forerunners were an advanced race that existed 100,000 years before the Halo series kicks off. They inherited the Mantle from the Precursors (tasked with the guardianship of life in the galaxy), they encountered the Flood on the planet G617-g1 whilst on a Pioneering excursion through the galaxy - after that, war broke out.
The Forerunners lost the war, they created defensive measures and eventually had to abandon worlds behind their Maginot Sphere (Earth was one such planet). They created the Halo Rings as a weapon of last resort (and as storage facilities), the Shield Worlds and the Ark.
The plan was to have the Didact activate Halo whilst the other Forerunners took refuge inside Micro Dyson Spheres in slipspace (Shield Worlds - Halo's harmonic frequency doesn't affect slipspace) so that the Flood starve to death and fall dormant. The plan worked, the Librarian passed down the Mantle to humanity and the surviving Forerunners exiled themselves from the galaxy to search for a new home because they failed to uphold the Mantle.
The Mantle is what makes humanity Reclaimers, it's why we're able to activate the Halo Rings (and other stuff) when the Covenant can't, we were chosen by them to reclaim their fallen empire.
/lesson