Time War time lock

Following the fall of Gallifrey, the events of the Last Great Time War between the Daleks and the Time Lords were contained by a time lock, thus isolating the conflict in its own specific timeline, (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) which was mostly separate from the normal universe. (PROSE: Revenge of the Nestene)

Origin
According to conflicting accounts, the time lock was placed by either the surviving higher species (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) or the Doctor. (COMIC: Don't Step on the Grass) This time lock was intended to ensure that no entity or faction could use time travel technology to revisit the war to alter or otherwise engage with any of the events that had occurred. Such interference was deemed too dangerous, likely to place the integrity of the Time Vortex itself at risk. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

In one account of the end of the Time War, the Eighth Doctor, having used the Moment to destroy Gallifrey Original, tumbled out of existence as the Time Lock containing the war solidified, falling into plasmaspace, then foulspace, then beyond into infinity before he was ultimately retrieved by the TARDIS. (PROSE: Doctor Who and the Time War)

At the end of the Time War the time lock tore right through the planet Lujhimene, severing it in half. The time lock maintained the planet's gravity and atmosphere, and also spun its core as if nothing had happened. The Eleventh Doctor visited Lujhimene looking for the Volatix Cabal. (COMIC: Running to Stay Still)

Breaking the lock
On the last day of the Time War, Lord President and his High Council understood that they were trapped inside the time lock. By transmitting a signal back through time and implanting it within the mind of the Master, who was fated to escape the Time War, the Time Lords had a link to 's location in the post-Time War universe, on Earth on 25 December 2009. By throwing a White-Point Star to Earth, Rassilon made the contact physical, resulting in Gallifrey briefly appearing in the sky above Earth, while Rassilon and a number of Time Lords appeared within the residence of Joshua Naismith. The Tenth Doctor warned that, with the time lock broken, it would enable the "hell" of the Time War to descend into the universe including not only the Daleks but the Skaro Degradations, the Horde of Travesties, the Nightmare Child, Could've Been King with his Army of Meanwhiles and Neverweres. Ultimately, the Tenth Doctor destroyed the star using Wilfred Mott's gun, forcing Rassilon and Gallifrey back into the war. Attacking Rassilon in retaliation for the impact the signal had on his mind throughout his life, the Master too joined the Time Lords in the war. (TV: The End of Time)

Just prior to the fall of Gallifrey, the War Doctor was preparing to use the Moment to end the war when the sentient weapon enabled him to make contact with his future incarnations, the Tenth and Eleventh Doctors in the post-Time War universe. The Tenth and Eleventh Doctors, along with Clara Oswald, were allowed to accompany the War Doctor back to the barn on Gallifrey. Knowing that the events were time locked, the Tenth Doctor acknowledged that they should not have been allowed to enter them, with the Eleventh Doctor understanding that they had been let through the time lock. It was after this that the Doctors and Clara decided to change the outcome of the war that they remembered, leading to "all thirteen" incarnations of the Doctor piloting their respective iterations of the TARDIS into Gallifrey's atmosphere as they relocated the planet into a pocket universe. After returning to the National Gallery with his future selves and Clara, the War Doctor took his leave in the TARDIS before regenerating into the Ninth Doctor, losing his memory of saving Gallifrey and so believing that it had been destroyed. (TV: The Day of the Doctor)

Dalek Caan, the last survivor of the Cult of Skaro, was able to enter the Time War after initiating an emergency temporal shift, proceeding to rescue Davros from death at the hands of the Nightmare Child. Together, they emerged in the post-Time War universe with Caan being greatly affected both physically and mentally by the ordeal, appearing to have gone insane to observers. Knowing that the "entire war" was time locked, the Tenth Doctor proclaimed Caan's feat to be "impossible" whilst Davros himself deemed it "remarkable" that a "single, simple Dalek" had succeeded where Emperors and Time Lords had failed. (TV: The Stolen Earth)

Time Sentinels which were left behind by the Time Lords grew concerned that the Tenth Doctor's actions could damage the Time War's time lock. They allied with the Red TARDIS and attempted to lock the Doctor in an alternate timeline. After the Sentinels were corrupted by the Red TARDIS, the Osiran Anubis made the Circle of Transcendence collapse, destroying the Sentinels. (COMIC: The Good Companion)