A Halo is an artificial, planet-sized ringworld weapon, created by the ancient Forerunner race as a weapon to combat the parasitic entity known as the Flood.
Despite millennia of scientific and philosophical advancements, the Forerunners were unable to stop the spread of the Flood as it propagated across numerous life-forms through infection. After many defeats, the only answer was to starve the Flood by denying its ability to infect anything - by destroying all sentient life.
The Ark was built to construct, maintain and activate the ringworlds that make up the Halo Array weapon, and seven ringworlds were launched at strategic points across the galaxy. The Halo Array was fired, wiping out all sentient life without damaging the inhabitants worlds and the Flood retreated.
However, towards the end of the Human-Covenant War, the Halo Array once again became an ever present menace, the weapon to end all wars.