You are proficient with bucklers, light shields and heavy shields.
Benefit: You can use a shield and take only the standard penalties.
Normal: When you are using a shield with which you are not proficient, you take the shield’s armour check penalty on attack rolls and on all Strength-based or Dexterity-based ability and skill checks.
Special: Barbarians, Bards, Clerics, Druids, Fighters, Paladins and Rangers automatically have Shield Proficiency as a bonus feat. They need not select it.
Source: Player’s Handbook (Page 100)