The Abyss is the name given to the super-massive Black Hole that makes up the core of the Galaxy.
Stars are very closely packed near the abyss making habitable planets very rare.
The gravity of the Abyss is such that it has a significant effect on the Warp as well as real space this effects warp travel making travel towards the Abyss much quicker than normal and travel away much harder. There are some 'calm' areas where transit is easier and these are valuable trade routes.
It is said that the power of the Abyss is such that daemons fear the area and even the Ruinous Powers themselves do not come near it.