WARNING: The following article contains major spoilers for Avengers: Infinity War, in theaters now.
Avengers: Infinity War has arrived and, as promised, it's a twist-laden rollercoaster of emotions. Packed with bombastic action sequences, fans get their fill of Earth's Mightiest Heroes teaming up with the Guardians of the Galaxy and the armies of Wakanda, all to fend off Thanos, who has finally decided to fill his Infinity Gauntlet with the Infinity Stones.
The Mad Titan eventually succeeds in his quest, using the combined power of all six Stones to reshape the galaxy by eliminating half its inhabitants. After barely surviving a fight with Thor, Thanos then teleports to an ethereal realm where he meets the younger version of his adopted daughter Gamora, painstakingly confessing that his mission cost him "everything," an admission made ironically to the 'daughter' he sacrificed by throwing her off a cliff in exchange for possession of the Soul Stone.
As the film ends, the final scene depicts a reclusive Thanos, overlooking a lush, hilly landscape and sunset with a smile on his face, content at what he perceives to be saving the galaxy. At first glance, it appears that he's on some new planet resting and recovering, but all signs point to it being the Soul World, a universe existing inside the Soul Stone. It also happens to be the perfect setup for Adam Warlock to officially enter the Marvel Cinematic Universe.
In the comics, the Soul World first popped in the works of creator Jim Starlin, alluded to in Strange Tales #179 in 1975, before actually being shown in Warlock Vol. 2 #6, eight years later. Basically, it was introduced as a pocket dimension that was housed within the Soul Gem (the pre-Stone name for the crystals), acting like a purgatory for souls. Initially, after being given the Soul Gem by the High Evolutionary, Warlock trapped the souls of his enemies inside it. As time progressed, he would develop a connection not just to the Gem, but to those residing inside it.
Warlock has also used the Soul World as a place of exile for himself, a sanctuary where he can find solitude to think and ponder the fate of the universe and his role in determining it. He's also used it as a haven for warriors he deemed to be righteous, and who needed a proper rest in the afterlife. A couple of them were Pip the Troll and Gamora, two allies that helped him in his early fights against Thanos, all to prevent the Mad Titan from collecting the Gems for his Gauntlet.
As for the MCU, well, many believe that Thanos ends up in the Soul World at the end of Infinity War, because when he's seen rising up from what seems to be a mystical body of water, he then goes to hold council with Gamora on whether his quest was worth it. There, a orange hue surrounds the duo in their vulnerable moment, a hue that matches the color of the Soul Stone Thanos recovered. Given that he killed Gamora and traded her life for the Stone, it would make sense that the version of her he loved -- his "little girl" to be more specific -- is the one that exists within this world, and is the person with whom he's conversing.