From Halopedia, the Halo wiki
Lagrange points are locations in space where gravitational forces and the orbital motion of a body balance each other. They were discovered by French mathematician Joseph Lagrange in 1772 in his gravitational studies of the 'Three body problem': how a third, small body would orbit around two orbiting large ones. There are five Lagrangian points in the Sun-Earth system and such points also exist in the Earth-Moon system.
Kepler's laws require that the closer a planet is to the sun, the faster it will move. Any spacecraft going around the Sun in an orbit smaller than Earth's will also soon overtake and move away, and will not keep a fixed station relative to Earth. However, there is a loophole. If the spacecraft is placed between a sun and a planet, the planets gravity pulls it in the opposite direction and cancels some of the pull of the Sun. With a weaker pull towards the sun, the spacecraft then needs less speed to maintain its orbit. If the distance is just right - about a hundredth of the distance to the Sun - the spacecraft, too, will keep its position between the sun and the planet.
A surveillance or imaging spacecraft at this point would not have to make constant orbits of the planet, which result in it passing in and out of the Planets shadow and causing it to heat up and cool down, distorting its view. Free from this restriction and far away from any heat radiated, it provides a much more stable viewpoint.
Commander Richard Lash, a UNSC naval officer, and captain of the UNSC Dusk was ordered to take his prowler to Onyx to provide the UNSC Stalingrad and its battle group with tactical data as well as to observe the battlefield. While taking high-resolution photos of the surface of Onyx, Lash ordered his sensors officer, Joe Yang, to scan Onyx's atmosphere from the planet's surface to the Lagrange points, where they then discovered two Covenant destroyers in high orbit.