The time safe was a feature found in various TARDISes, commonly stored behind a roundel in the console room.
A time safe was used to provide a Time Lord with something from the future that would be needed at that point in the TARDIS's relative past. The Fifth Doctor described it as a permitted temporal paradox that must be used sparingly.
When the safe was used, the past safe generated an urgent beeping to draw attention to itself. It manifested behind one of the roundels in the control room, opening onto a mirror surface until the initial stasis effect wore off and the future object arrived in the present.
The Doctor used it to send himself the diary of Captain Richard Haliwell regarding the British expedition to the Moon in 1878, having received the diary from Haliwell after the decision was made to suspend the British space program, alerting the Doctor and Turlough to their role in the expedition. (PROSE: Imperial Moon)