The Gellar field is a form of protective energy bubble that shrouds a starship as it travels through the warp, the roiling parallel dimension that vessels must enter to cross the vast distances between stars. The immaterium is home to predatory entities and daemonic horrors, and any ship exposed to its raw currents would be swiftly overwhelmed and its crew driven mad or devoured. The Gellar field holds back that nightmare, enclosing the vessel in a sphere of stable reality within which the crew can survive the passage.
Unlike void shields, which turn aside physical attacks, the Gellar field exists to keep the horrors of the warp at bay, a fragile membrane between the material and the immaterial. Should it flicker or fail, even for a moment, the consequences are catastrophic: daemons pour through the breach, and the ship becomes a slaughterhouse or is lost forever amid the tides of the warp.
Because so much depends upon it, the Gellar field is maintained with obsessive care by the tech-adepts of the Adeptus Mechanicus, its generators tended through constant ritual and vigilance. Every warp voyage is a gamble that the field will hold, and tales abound of ghost ships found drifting with their crews torn apart, silent testament to a Gellar field that failed in the depths of the immaterium.