Why are three of saturns satellities called shepherd moons?
Their gravitational effects and distances from the rings causes matter at certain orbital distances from Saturn to be eliminated, making Saturn's rings more well defined.
Shepherd moons are those which are close to the rings of a planet. Their gravity effects the material which makes up the ring, causing well defined bands and edges to form in the ring system over time. Ring matter will either collide with the moon and become part of it, or it will either be deflected away into space or back into the ring.