Sin is an effect caused by killing non-hostile characters in Dark Souls III.
Sin is accumulated as the player kills non-hostile NPC's in the Dark Souls world. As the player's sin rises through these actions, certain game mechanics begin to change. For instance, the Shrine Handmaid will begin charging more souls for items that otherwise would have been normal costs.
To prevent sin, a player must simply not kill any supportive or non-hostile NPC (with the exception of Crystal Lizards).