Mordo: Temporal manipulations can create branches in time. Unstable dimensional openings. Spacious paradoxes! Time loops!!! You want to get stuck reliving the
same moment over, and over, forever, or never having existed at all?
Dormammu: You... what is happening?
Strange: Just as you gave Kaecilius powers from your dimension, I've brought a little power from mine. This is time. An endless looped time.
Before Strange confronts Dormammu, he uses the Eye of Agamotto to create an unbroken time loop. This is represented by the glowing green "bracelet" that he creates and wears. The result is Strange and Dormammu (and possibly everyone in the world) being stuck reliving the same short window of time encompassed by the time loop Strange has created.
Strange: Take your zealots from the Earth. End your assault on my world. Never come back. Do it, and I'll break the loop.
Once Dormammu has agreed to Strange's demands, he presumably uses the Eye of Agamotto's powers to break the time loop, and the bracelet is not seen again.