The Apostasy Gambit
The technology that generates a protective Gellar Field (also spelled as Geller Field) around a starship intended for faster-than-light travel was invented along with the Warp-Drive sometime in the 15th Millennium AD during the Dark Age of Technology. The technology, called a Gellar Field Device, allowed Warp-capable starships and their occupants to survive the extremely hostile environment of Warpspace, also known as the Immaterium. The Gellar Field protects the starship and its occupants from the hostility of the psychically-reactive Warp itself as well as from the predation of Warp entities such as daemons.
The Gellar Field Device emits an energetic, invisible force field comprised of unknown subatomic particles called a Gellar Field, which essentially maintains a bubble of real space-time around a starship traveling through the Warp using the power of its sublight drives after using its Warp-Drive to pierce the veil between the physical universe and the Warp. Since entities native to the Immaterium cannot breach its boundaries to enter the physical universe except under certain very specialized circumstances, creatures native to the Warp are repelled from the boundaries of the Gellar Field. The weakening, failure, or collapse of its Gellar Field while a starship is traveling through the Warp would be completely disastrous, as was discovered by the crew of the Imperial frigate Eisenstein during its famous flight to Terra from the Battle of Istvaan III to tell the Emperor of Mankind of the betrayal of his favoured son Horus. Warp entities would tear the starship apart to reach and consume the souls of the crew. As such, the field must be active at all times. For the field to work, the starship must be entirely sealed; no open entrances that would breach the field’s protective envelope are allowed.
From Warhammer 40k Wikia