Original: The gravity of shepherd moons serves to maintain a sharply defined edge to the ring; material that drifts closer to the shepherd moon's orbit is either deflected back into the body of the ring, ejected from the system, or accreted onto the moon itself.
Simplified: Any material that drifts closer to a moon's orbit, it can either move back into the body of the ring, deflect out into space, or is added onto the moon's surface.