This line is spoken by Thanos to Doctor Strange during the battle on Titan.
He is, of course, referring to the Time Stone and he has a very good point. The Sorcerer Supreme used the stone to defeat Dormammu but fans have already concludede that Strange had seen the one and only path to defeating Thanos and so allowed events to takeplace as they did.
However, this does not mean he wasn't using the Time Stone anyway. In fact, watch the scene again and there is ample proof that he actually pulled the Stone from another point in time. Watch the expressions on both Thanos and Strange's faces.
Was the Stone in the future? What was it doing there? Will it appear to Tony Stark at the very start of Avengers 4?
A remarkably strong theory from Redditor Lol-Guru explains everything.
"Dr Strange conjuring forth the stone the same way as Loki did the tesseract is a gigantic misdirect! Loki is able to conceal the tesseract with his godlike powers of illusion and while Dr Strange could certainly be capable of mimicing this easy trick, I don't think that's what's happening. The stone is glowing brilliantly because it's actually travelling back in time from the future!
"When Dr. Strange hid the time stone it wasn't through some trick of light or illusion like Loki but by sending it forward in time to be used later. Only to be sent back when the Avengers had finished using it."
"The time Stone can travel forward or backward in time, but not space. It will appear on Titan soon after the snap presenting itself to Tony.
"This is also why Tony needs to survive. If the snap is inevitable then he is the only one smart enough to figure out what is going on when the time stone presents itself on Titan and study the stone in order to unlock the quantum realm (the same way the mind stone in the sceptre allowed him to create Ultron).
"This is why the Gauntlet breaks from the snap! It's using a time stone from a reality where the Avengers have already won and sent it back in time. The paradox of using a stone from a mutually exclusive reality breaks the gauntlet and helps advance Dr. Strange's plan even further because it cripples Thanos' ability to fight back when the Avengers start mounting their counter offensive."
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It is a powerful and coherent argument, strengthened by the intense look on Strange's face as the Time Stone floats over to Thanos. Strange knows this moment is necessary but is he is also worried Thanos might spot something fishy is going on? Furthermore, Thanos does actually shoot a suspicious look across at him.
Of course, the Gauntlet might have been damaged simply from channeling such an extraordinary amount of power but this is certainly one of the most convincing and plausible theories.