You are skilled at wearing light armor.
When you wear a type of armor with which you are proficient, the armor check penalty for that armor applies only to Dexterity- and Strength-based skill checks.
A character who is wearing armor with which he is not proficient applies its armor check penalty to attack rolls and to all skill checks that involve moving.
All characters except monks, sorcerers, and wizards automatically have Light Armor Proficiency as a bonus feat. They need not select it.