An orbiter is a small companion, always closely circling around your ship or travelling on its hull. Friendly types of orbiters are able to help the ship's pilot in various tasks or ways. Should your ship be destroyed or should you change your ship while an orbiter is installed, the orbiter will be lost (regardless of whether an escape pod is installed or not).
All orbiters are avalilable as commodities. To install one on your ship, you have to carry one and press [Inst] next to its icon in the cargo display on the nav screen.
At the same time, only one orbiter can be installed on your ship! It is possible to remove an installed orbiter any time in the Overview->Ship Screen.