From Halopedia, the Halo wiki
Planets are celestial objects in the Milky Way galaxy. They are found in systems orbiting stars. Planets themselves are sometimes orbited by both natural satellites and artificial space stations. Cities are also often built on planetary surfaces.
Terraforming is a series of processes to make planets suitable for habitation. It can take several centuries to fully terraform an Earth-sized planet, depending on what is needed. Humans first began terraforming around the 22nd-century in the Sol system. The Covenant can terraform worlds solely to produce plasma. The Flood could also terraform, by altering the atmosphere of worlds they touched.
Planets can generate enough of a gravitational force that it can affect the superfine quantum filaments that are needed for slipspace drives; something UNSC calculations couldn't figure out. Covenant drives, which originate from Forerunner drives aren't affected by this.