A time-lock is a mechanism whereby an event or series of events is rendered unreachable by time travel. The time-lock only prevents entry into the Time War through the time vortex itself, meaning all the events were not erased from history, and thus can still be referenced to by those who knew of the destruction of the Daleks and the Time Lords. (Such as the reference made in Bad Wolf). The Doctor, however, would only be able to reach the Time War through time travel, and any entry into the events of the Time War would be considered a change in the time line (thus made impossible due to the time-lock).
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). It is known that the Time Lords strictly forbid any change, travel, even observation into past or future events of Gallifrey. Thus, it is likely that the Time Lords put the time-lock in place immediately, for they knew that time-travelers would be constantly changing the events of the Time War, which could be extremely dangerous to the time line. 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)
Another sort of time-lock, created by Torchwood 3's resident genius Toshiko Sato, is designed to freeze the Torchwood 3 Hub in a bubble in time, which is at the same time impenetrable but inescapable. (DW: Journey's End)