The Null Zone was a part of the universe where time travel was impossible. The Null Zone was generated by a Time Lord weapon from the Omega Arsenal given to the Daleks by Seratrix. Contained in the null zone was the group of planets called the Thousand Worlds, where the Daleks and Taalyens enslaved the inhabitants to drill to the core of the planets in order to destroy Gallifrey.
Seratrix was captured by the Daleks after he delivered the Time Lord technology. Later, Cardinal Ollistra sent a team consisting of the War Doctor, Veklin, Bennus and Arverton to rescue him. During this mission, the Doctor discovered about the plan of the Daleks to drill into the core of the planets and found that the central control for the operation in the whole Null Zone was on the planet Keska. After stopping the drills, the Null Zone was deactivated. (AUDIO: The Heart of the Battle)
However, after the Time War, the Tenth Doctor, Gabby Gonzalez, and Cindy Wu travelled via a space bus through the Null Zone, situated in deep space, in the 24th century, the Doctor noting that there were some areas where "time travel's a bit tricky these days." (COMIC: Supremacy of the Cybermen)