Threshold is the name of the large, orange colored planet seen in the demo hologram room sequence. According to a Bungie preview quote, it is a large gas giant planet, and the ringworld orbits it on a halo (or lagrange) orbit between it and a smaller moon planet.

One of the possible connections between Halo and Marathon is the idea that the Halo is of Jjaro (or Jjarro) origin. This race had the ability to move entire moons through space.

One of the possible purposes of the Halo itself could be as such a transportation system. The name of the nearby planet Threshold suggests a place nearby may be a doorway...