Directly extracted from the wiki.
Local SymmeLocal Symmetry allows you to switch the pivot point used for symmetry to be the center of the visible mesh.
When you move a SubTool off the main axis of the model you can use this option to still sculpt in symmetry.
Retrieved from "http://www.zbrush.info/wiki/index.php/Local_Symmetry"
It's pretty clear to me, and pretty useful.