After the shuttle has completed its mission, to return back to earth it has to deorbit- i.e. free itself from the influence of planet's gravity.
In returning home, the orbiter must be sufficiently decelerated by an OMS (Orbital maneuvering system) so that when it enters the atmosphere, it maintains control and glides to the landing site. For the nominal end of mission, a retrofiring of approximately 2.5 minutes is performed at the appropriate point in the vehicle's trajectory. For this maneuver, the orbiter is positioned in a tail-first thrusting attitude. Deorbit thrusting is nominally accomplished with the two OMS engines.
About one hour before deorbit, the crewmembers take their seats. The spacecraft is then manually maneuvered to the deorbit attitude. About 30 minutes before deorbit, the OMS is prepared for deorbit thrusting.
The deorbit phase of the mission includes deorbit preparations, execution and monitoring, and maneuvering to approximately 400,000 feet. This point is called the entry interface.