It's confirmed the Reality Stone will play a part in The Avengers: Infinity War, which is one of the six stones that make up the all powerful Infinity Gauntlet.
Screen Rant caught up with The Avengers: Infinity War director Joe Russo who offered:
“Each gem has its own special power. We had to put a lot of thought into how we would represent those powers so it was clear to the audience. Not only are you dealing with a lot of characters, you are also dealing with the stones, the gems, and there’s a lot of them. So for people to keep track of all of it, we had to get really specific. I won’t divulge how we use the Reality Stone, but we do use it.”
The Reality Stone was first seen in Thor: The Dark World as the Aether, yielded by Malekith. Odin described it as: "While the other relics often appear as stones, the Aether is fluid and everchanging. It changes matter into dark matter. It seeks out host bodies, drawing strength from their life-force."
The Reality Stone is currently housed in the Collector's Museum, but that may change with Avengers: Infinity War as the leaked Avengers: Infinity War trailer may have revealed the Collector's Musuem in ruins. The Reality stone was also featured briefly in Thor's dream during The Avengers: Age of Ultron.
In the comics, the Reality Gem is colored yellow and is described as (via wiki):
Allows the user to fulfill wishes, even if the wish is in direct contradiction with scientific laws, and do things that would normally be impossible. The gem can also be used to create any type of alternate reality one wishes and when backed by the other stones it allows one to alter reality on a universal scale. Occasionally stated to be the most powerful of the Gems, if wielded without the other Gems to keep its reality-altering powers from accidentally surging out of control. At one point, the gem rapidly cycled through multiple alternative universes.
It's unknown how the Reality Stone comes into play in The Avengers: Infinity War, maybe the heroes use it against Thanos?
The Avengers: Infinity War hits May 4, 2018 directed by Joe and Anthony Russo.
As the Avengers and their allies have continued to protect the world from threats too large for any one hero to handle, a new danger has emerged from the cosmic shadows: Thanos. A despot of intergalactic infamy, his goal is to collect all six Infinity Stones, artifacts of unimaginable power, and use them to inflict his twisted will on all of reality. Everything the Avengers have fought for has led up to this moment - the fate of Earth and existence itself has never been more uncertain.