The Boundary was a portal on the shore of an unnamed planet in the far future, activated by the proximity of humanoid people. It worked as a gateway, a direct route into another random part of the universe.
The Boundary patterns were said to never connect to the same place twice so it was used by the humans during the late Cyber-Wars to escape the Cybermen. It was guarded by Ko Sharmus, who helped the fugitives to get through it after having located the Boundary himself while fleeing a Cybercamp.
Eventually, it worked as a gateway to the Gallifrey ravaged by the Spy Master. The Thirteenth Doctor believed that the Master was responsible for this, though the Master refused to explain how he had connected the Boundary to Gallifrey. (TV: Ascension of the Cybermen, The Timeless Children)