Kaecilius and the zealots have killed the sorcerers at the Hong Kong sanctum and destroyed its beacon; with the beacons at London and New York already broken, Earth has no protection against the Dark Dimension, leaving Dormammu free to invade.
Strange and Mordo arrive and witness the destruction before Strange uses the Eye of Agamotto to rewind time. The pair resurrect Wong, but Kaecilius halts the spell right before the beacon is rebuilt, freezing the group in time.
As Kaecilius describes the Dark Dimension as a reality beyond time, Strange enters and traps himself and Dormammu a time loop; Strange is repeatedly killed, but time resets to the moment Strange arrives. As time does not exist in the Dark Dimension, Dormammu has no control over it. His will is eventually broken and lets Stephen name his terms for the end of the time loop; to leave Earth alone, never return, and to take Kaecilius and the zealots with him.
Time begins to flow in Hong Kong as the rogue sorcerers are dragged into the Dark Dimension. Though the day has been saved, Mordo is appalled by Strange using the reality-bending time magic and abandons the sorcerers. Strange and Wong return to Kamar-Taj, where it is revealed the Eye of Agamotto's time controlling nature is due to the fact it contains an Infinity Stone. Strange is appointed master of the New York sanctum as he accepts his role in the mystic arts.
In a scene set during the credits, Thor visits Strange and asks for his help in finding Odin; this hints at the events in Thor: Ragnarok. In a scene set after the credits, Mordo tracks down Pangborn and absorbs his magic, reverting Pangborn to a quadriplegic state.
Answer: I took it to mean Dr. Strange could hear the watch ticking, and he wanted complete silence.
Bishop73