SRD3e:Armor Proficiency (light)
From D&D Wiki
You are proficient with light armor
Benefit: When you wear a type of armor with which you are proficient, the armor check penalty applies only to Balance, Climb, Escape Artist, Hide, Jump, Move Silently, Pick Pocket, and Tumble checks.
Normal: A character who is wearing armor with which she is not proficient suffers its armor check penalty on attack rolls and on all skill checks that involve moving, including Ride.
Special: All classes except wizards, sorcerers, and monks have this feat for free.