http://www.creationkit.com/Glossary#Depth_Bias
Depth bias (on shadow lights) is a method to account for insufficient depth precision in shadowmaps. Small depth bias can result in striping artifacts on the surface of objects, large depth bias can result in a lack of contact shadows.
I suggest you watch this http://www.youtube.com/watch?v=5fZoIpKcJ6I&feature=BFa&list=PLD5AA9F15CAA68B07 by Bethesda.