Adds damage to a successful attack role. The opponent must be flatfooted or flanked otherwise the sneak attack does not deal extra damage. Two rogues can flank an opponent and use two sneak attacks in succession.
Cade sneaks (hides/moves silently) behind the Ogre and rolls a d20 for his attack. He gets a 17 for a total of 23 which easily beats the Ogre's AC.
He then rolls a 1d6 for an attack with his short sword and adds 2d6 damage. He gets the maximum damage of 18.