The Infinity Stones are key components in the Marvel Cinematic Universe, popping up across space and time and wielded by numerous characters towards different ends. Each stone embodies a different facet of elemental existence, containing immeasurable power. They were ultimately united in the worst way possible during Avengers: Infinity War, when Thanos uses their combined strengths to wipe out half the life in the Universe. A number of fan theories about what will happen going forward with Avengers: Endgame, out April 26, hinge heavily on the Time Stone, so how does the Time Stone work, and what does that mean for the future of the Avengers?
First, a little backstory. As per usual with comics, there's conflicting information within the same universe as to timelines and history (ironic, as a Time Stone would really help sort this straight). In Doctor Strange, Wong says the stones were created by/with the Big Bang and sent hurtling across the new universe. Other Marvel comics and an explanation by the Collector in Guardians of the Galaxy 2 stick with the story they were created by Cosmic Entities Entropy, Infinity, Eternity, and Death, four primordial beings who balance each other out, and one of whom Thanos has a crush on — he might've said he planned to dust half the universe to save it from overpopulation, but in the comics at least, it's done to impress Lady Death.
So each stone controls a basic aspect of reality, and to a certain extent is that element of reality. What does that mean for the Time Stone? Well, as we saw in Doctor Strange, the Time Stone can manipulate time - reversing it, creating loops, slowing it down or speeding it up.
It also is time; Strange ultimately defeats Dark World enemy Dormammu, who exists in a space outside temporality, by bringing time to him using the stone. The stone itself is the element of time.
Sidebar — take a closer look at that gif — it's pretty easy to spot the Time Stone in the Eye of Agamotto (the amulet Strange wears that houses the stone), because it's glowing. That's how you know a stone's being used. Normally inert, they'll begin to glow brightly once activated.
Here's Strange powering that bad boy up again:
Before use: very shiny, but not glowing. DURING use: glowing. And look at that, what happens as Strange hands over the stone to Thanos:
Hmmm, very interesting. But, back to the stone's use. It's a time manipulator, it is the embodiment of time, and as such, exists through time simultaneously, giving any user control over past, present, and future. According to the Marvel Fandom site, the stone can act as portal to its own past and future points, along with giving its user ability to travel along them, and when used in conjunction with the other Infinity Stones, gives its user a similar ability to exist at all points in time simultaneously — a form of omnipotence. It could also render any user effectively immortal, as it gives them control over personal temporality, including aging.
Of course, this requires mastery of the stone's powers. Anyone who's not an expert in time manipulation wouldn't be able to do much with it. In fact, to keep the Time Stone as safe as possible post-Thanos, in the comics it was given to a reinstated Gamora who specifically said she didn't know how to use it, but was strong enough to protect it from anyone who did, a double-safety. Meanwhile, immediately after handing the stone over to Thanos in what seemed like the world's worst trade-off, Doctor Strange confidently states that they are all now in "endgame." What exactly he meant by that will remain a mystery until the Avengers movie of the same name hits the screen, but odds are it'll have something to do with the powers of the Time Stone.