The Gellar field was invented along with the warp drive, allowing warp-capable ships and their occupants to actually survive the hostile environment of the warp. It protects the ship and its occupants from the hostility of the warp itself as well as from the predations of warp entities such as Daemons. Gellar Fields are generated via a comatose Psyker slaved into the bowels of Imperial vessels.
The Gellar device emits a wave called a Gellar field, essentially creating a bubble of real space around the ship. The weakening, failure, and collapse of a Gellar Field while the ship is traveling through the warp would be completely disastrous. Warp entities would tear the ship apart to reach and consume the souls of the crew.