In Trek it's a low-level forcefield similar to the Structural Integrity Field distributed throughout the vessel via the grav-plate system where the field is distorted in a directly opposite vector to the direction of acceleration. Though there is a lag from acceleration to counter the force and this is especially noticeable on unexpected movements.
Atlas either does not explain the underlying system, has some similar techno-babble explanation or the acceleration is sufficiently slow enough to not effect those on-board. I doubt the latter as you'd probably not be asking if that was the case.
Hope is but the first step upon the road to disappointment.