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.
Many classes and variants of Gellar fields exist , and forge worlds specialising in field technology such as Belecane (in the Calixis Sector) fabricate warp drives alongside stasis fields, Void shields, and other protective barriers. Some Gellar field generators used during the Great Crusade use a comatose Psyker to generate a Gellar field. 
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.
Gellar Fields can be disrupted through technological means, such as through use of a Hades-Pattern Rotating Gravimetric Harmonic Disruption Device.
- 1: Codex: Chaos Daemons (4th Edition), pg. 43
- 2: Ashes of Prospero (Novel) Chapter 6
- 3: Rogue Trader Core Rulebook, pg. 199
- 4: Rogue Trader: Battlefleet Koronus, pg. 32
- 5: Dark Heresy: The Lathe Worlds, The Lost Dataslate, pg. 11