Doctor Strange: 31 Easter Eggs & References You Must See

As suspected before the film's release, it is revealed eventually by Wong that the Eye Of Agamotto is actually an Infinity Stone.

Specifically, it is the Time Stone, able to command power over the progression of time and reality. It allows the user the vision of possible futures, time travel and control over the age of beings. It can also be used as a weapon by trapping enemies or entire worlds in unending loops of time, as Strange does with Dormammu when he meets the creature.

That should have been enough for fans to recognise the artefact, but Wong later confirms as much, saying it wouldn't be sensible for Strange to walk the streets with one hanging around his neck. There's even an implication there that he's aware that Thanos is seeking them.