Within the Halo series of computer games and books, the Forerunner are an intelligent, interstellar species within the Halo series. They are a space faring, mostly peaceful race spanning the milky way galaxy.  They have achieved "Tier 1" ranking on their own Technological Advancement scale. It measures a race's technological prowess. Going from 7, being the worst; to 0 being the best. Only one species has surpassed the Forerunner on their scale, a species long dead called the precursors. Not much information is known about them aside from the fact that they can travel between galaxies and dimensions. 
Technology and Engineering
To attain Tier one: world builder status the Forerunner have became masters of the universe around them. They have the ability to manipulate space/time and make extremely accurate slipspace jumps among the milky way galaxy. They have the ability to manipulate gravity to their free will. They can create artificial intelligence with complete sentience. But most importantly they have the ability to create life and worlds. They are notorious for their mega structures such as the Halo arrays. Many of these structures are large enough to support an entire ecosystem and mimic naturally created terrain and atmosphere.
The forerunner consider themselves to be the keepers of "The Mantle". A title believed to be passed down to them from the precursors. The role of the keepers of the mantle is to ensure intelligent life thrives within the galaxy no matter the cost. The Forerunner were mostly peaceful and were even vegetarian by choice in order to not consume any species under their care. However, they were extremely advanced and an incredibly formidable foe if provoked. They are the creators of single most lethal weapon ever known in the Halo universe. The Halo array. A set of super weapons meant to destroy all life residing in the milky way galaxy.
- Buckell, Tobias S. Halo Encyclopedia: The Definitive Guide to the Halo Universe. London: Dorling Kindersley, 2009. Print.