This is a skill that makes it so that when you hit a enemy with a arrow, it does not knock it back, knock it down, or make it so it even got hit. This makes it so that you can support with damage, but not interfere with another's attacks. If you kill an enemy with this skill, and anyone else attacked the enemy, they will get the exp instead. This skill as mainly to help another person, and not kill steal them.
You get this skill by talking to Trefor about skills after getting the Ranged Attack skill. Once you get the skill, it ranks up to rank F on its own.