The Soul Gem has been an enigma for almost all of the history of the MCU. It's barely even been mentioned, much less seen, until it finally made an appearance in Infinity War. However, since it was one of the last Stones to make an appearance in the movie, it's hard to tell what specific powers it has.
Based on the events surrounding its procurement, I would assume it has powers over life and death themselves. Specifics would be nice, though.
I prefer some sources from the Avengers movies themselves, but comic references work too.