Gellar Field
The Gellar Field is a energy field that protects a space ship, oneself, and crew from getting a Class 10 Clusterfucking from Chaos while traveling in the Warp.
As everyone knows, the only way to get anywhere without taking fucking forever is to use the Warp. That is, unless you are the Eldar, in which case you can go much faster with Webways as long as you don't get lost. Or you are the Tyranids and can use gravitational Star Trek bullshit. Or you are the Necrons and can screw the laws of physics seven ways till Sunday. Or you are the Tau who invent new and less-shitty-then-previous but still-shitty-non-Warp-limit-as-x-goes-to-the-speed-of-light drives like every 20-30 years. So maybe not everyone knows. But everyone definitely knows that all sorts of fucked up shit lives in the Warp, whose ONLY goal is to fuck you up (both literally and metaphorically) in 57 (or more) different variations. The Gellar Field prevents this from happening, so the Servants of the Imperium can go to your planet and kill your Chaos-worshiping ass!
Let's Take an Example
The Orks are a fantastic test case of what happens when someone who doesn't use Gellar Fields travels through the Warp.
The Orks can use a similar device to a Gellar Field, that focuses the power of the WAAAGH! into an energy field to keep the Daemons and other warp monsters away. It involves jamming Really Big Teef onto the ship in an effort to frighten passing Warp-monsters. Hell yeah! This may or may not be a kind of ferryman payment to Gork or Mork for safe passage. Of course, if that technique doesn't work, the Orks will usually just give the unlucky daemons that tried to board their spacecraft a good stompin'. Either way, the Orks like both outcomes.
Many grassroots WAAAGH!s get underway by hitching a ride on a passing Space Hulk. These are convenient because they can enter and exit the Warp (saving on having to make a very complicated Warp Drive), have tons of cargo space (for all of the boyz to ride in), and frequently come equipped with friendly passengers such as Genestealers, Daemons, Chaos Cultists, or other some forgotten horror (for in-flight entertainment). This travel method, while common for Orks, would be utterly disastrous for any other race. Space Hulks have no guidance systems: they can spend decades in the Warp and come out right where (and when) they started, or spend a few days in the Warp and arrive hundreds of years in the past around a totally uninhabited planet. While all of this waiting around is happening, the totally unshielded Hulk is ripe for invasion from Daemons, who not only look at Realspace beings as a food source but are actively drawn to anything Real that happens to pass by. Brainsuckers, sanity-feeders, life-drainers, and eight-million copies of Radical Larry are on standby to slither into the Hulk and track down any Ork stupid enough to have hitched a ride in the first place. All of that would be bad enough, but because the Warp and Realspace don't mix well, Space Hulks are constantly in danger of of doing things like phasing into themselves, having infinitely-looping corridors and generally getting all M.C. Escher.
It's lucky for the Orks that they don't care much. Any other race would be driven mad, slaughtered entirely, and/or be devastated to learn that they arrived decades too late to stop the conflict they were sent to fight. Orks by contrast will readily fight anything and everything, including themselves (and terrifying horror-movie antagonists) as a general first response to unpleasant or unfamiliar circumstances. Due to their below-average intelligence they generally take eldritch horror with a puzzled chin-scratch and eventual dismissal (or assault and battery). Ork WAAAGH!s are also fairly independent of things like nations or strategic timelines, so "showing up too late" is almost always a non-issue. (In one instance the Orks in question showed up too early, catching their own WAAAGH! right before it left. Long story short, the Warboss killed himself to get two of his favorite gun and the WAAAGH! stopped in it's tracks) The above is not to say that Orks who hitch a ride on Space Hulks are perfectly fine. They aren't. Many would-be WAAAGH!s begin and end when the horde jumps aboard a Space Hulk, and it shows up on the other side of the galaxy with a few thousand new sets of Ork skeletons inside.