The Soul Stone is a little bit of a mystery. It wasn't revealed up until Thanos ended up sacrificing Gamora, who we know right now as one of the only things his cold heart ever loved (in his own unique way). Though Thanos wakes up with the Infinity Stone in-hand, we're not quite sure how he procured the Soul Stone to carry out it. Seeing that the movie easily skipped over this part, some
theorize that Gamora didn't perish, although alternatively
became the Soul Stone. Case in point, half of the universe that Thanos "killed" would be trapped indoor within the stone. Because like its name implies, the Soul Stone harbors souls.
The Soul Stone is mentioned to be sentient, and can trap lost souls indoor in a global that feels, looks, and seems for all intents and purposes real. This is the Soul World, a plane where a character named Adam Warlock, who has although to prepare his debut in the Marvel Cinematic Universe, dwelt for a long time in the comics. He eventually even brought Gamora there — so it could make brilliant sense that not only is Gamora in the Soul World, yet so are the rest of the people that faded into dust.