You're on it's bloody wiki, try doing some research rather than asking! Dumbass.
to the retard above me: he said hal not halo
HAL is either Hindustan Aerospace Ltd., the lead aircraft manufacturer in India, or HAL 2000, the AI from the 1971 film 2001: A Space Odyssey. This isn't wikipedia, so don't ask.
Pretty sure the guy meant to ask, "What is Halo?" If so, the Halo Array was a series of ring-worlds designed to destroy all sentient life in an effort to stop the Flood, a parasitic species that sustains itself by feeding on sentient creatures. The seven Halos were scattered across the Milky Way to maximize their effect. Each ring has an effective range of 25,000 light-years, and if all of the Halos fired simultaneously, all intelligent life in the galaxy would be annihilated. The Halo Array was constructed by the Forerunners, an incredibly-advanced alien race that existed over 100,000 years ago, during the final years of their war with the Flood. With time running out, the Foreunners built the Halo Array in a desperate effort to destroy their enemy. As the Flood neared the Maginot Sphere, the hypothetical border between what the Forerunners could save and what they could not, the Forerunners collected samples of all of the sentient creatures in the galaxy and stored them in places that would be shielded from the Halo Effect. With 4.8 million Flood ships storming into the Maginot Sphere, the Halo Array was fired, destroying the Flood, Forerunners, and all other intelligent creatures for millions of light-years. After the blast, the AIs began to carry out their purpose of re-seeding the galaxy of sentient life, including Humans, Elites, and Prophets, to name a few. For more information, look up "Halo Array" on Halo Nation.