Time lock

A time-lock is a mechanism whereby an event or series of events is rendered unreachable by time travel.

It is known that after the Last Great Time War, the entire war was time-locked (although it is not made clear whether the lock was in place during the war itself). Davros suggested that emperors (possibly Dalek Emperors) and Time Lords had attempted to breach a time-lock, but failed. Dalek Caan was able to overcome the lock on the Time War after making an emergency temporal shift, and travelled back to the Gate of Elysium to save Davros. The process drove Caan insane, but also seemed to grant him precognitive abilities. (DW: The Stolen Earth)

The reference made by the Doctor to the Time War being time-locked could also be referencing a previous comment that he had made with regards to certain events in the timeline being fixed points in history. (DW: The Fires of Pompeii)