The Immaterium is Mankind's solution to the problem of faster-than-light travel. The Warp functions as the medium for FTL interstellar travel, with voidcraft entering it through the use of a Warp-Drive. They then use their own mundane reaction drive systems and the aid of a psychic Astrogator to navigate between its flowing currents of psychic energy, as if they were moving through an ocean.
Warp travel is more art than science, somewhat unpredictable and always dangerous. Voidship may arrive at the wrong place, at the wrong time or be lost in the Warp and fell prey to Daemons.
Imperial Astrogators follow the light of the Astronomican. Chaos worshippers use sorcerers and Daemonic guidance.
In order to enter the Warp, voidship must first reach the star system's Mandeville's point — zone typically on the edge of star system, then activate the ship's Warp drive.