Your mastery of the shield allows you to fight with it without hindrance.
Benefit: You do not suffer any Two-Weapon Fighting penalties on attack rolls made with a shield while you are wielding another weapon. Add your shield’s enhancement bonus to attack and damage rolls made with the shield as if it were a weapon enhancement bonus.
This only applies to Two-Weapon Fighting since FAQ, originally applied to all penalties.
Pathfinder RPG Core Rulebook. Copyright 2009, Paizo Publishing, LLC; Author: Jason Bulmahn, based on material by Jonathan Tweet, Monte Cook, and Skip Williams.