"Do you really wish to know?" — Spoilers from the books and/or adaptations to follow!
Yorg was a man that, in 1272, was chained up to a rock in Skellige, having been accused of killing his uncle, Gretter, after his sister told him Gretter had molested her. However, this was actually a lie and his sister simply didn't like the uncle and thought he stank, not realizing Yorg would kill him.
If Geralt frees him: he will thank the witcher, promising to repay him should they ever cross paths again.
If Geralt leaves him to his fate: Yorg soon died from the punishment he was sentenced to.