You are trained in how to properly use a shield.
Benefit: When you use a shield (except a tower shield), the shield's armor check penalty only applies to Strength- and Dexterity-based skills.
Normal: When you are using a shield with which you are not proficient, you take the shield's armor check penalty on attack rolls and on all skill checks that involve moving.
Special: Barbarians, bards, clerics, druids, fighters, paladins, and rangers all automatically have Shield Proficiency as a bonus feat. They need not select it.