The Glitch speaks of both the event that brought about the end of the world, and the hole in the universe itself.
The event was caused by Flowey's theft of the six human souls, altering the world with his imperfect ascension to godhood. His hold on the magic of the souls was unstable, breaking from his monstrous form in arcs and bolts, and in his ineptitude and ego, he ripped a physical tear in the space-time continuum, allowing the entrance of the Void itself into the Underground. The mists spill in constantly, confined to the caverns belowground by the Barrier, and brought about the rise of the Hunger.
The only way, it has been hypothesized, to defeat the Glitch and close the hole in the universe, is to relieve Flowey of the souls in his possession.