A spatial vortex is a physical passageway linking two points in space, which can allow transit from one point to the other to any vessel passing through.
By 2375, a spatial vortex was discovered by the Malon Waste controller Emck, leading to "the Void," a region of the Delta Quadrant wherein no star systems existed within 2,500 light years. This aided his business of dumping antimatter waste of his civilization there with his Malon freighter, cutting his transport expenses in half; but with the consequence of severely damaging the health of the only native inhabitants.
The crew of the USS Voyager also learned about the vortex, and the existance of the natives. In order to both protect the aliens and exit the Void, Captain Janeway developed a plan whereby photon torpedoes would be used to destroy the vortex. Doing so, however, would require collapsing it from the end of the vortex located in the Void, where the vortex's dimensional radius was weakest. Since this act would effectively strand Voyager in the Void, the crew modified the plan to deploy the torpedoes immediately upon entering the vortex, then jumping into high-warp to outrun the shock wave. Voyager's warp drive was subsequently disabled in combat with the Malon, however, so Janeway ordered their aft shields reinforced via their main deflector, allowing the ship to ride the shock wave to the other side of the vortex. (VOY: "Night")