You are proficient with medium armor (see the Armor and Shields table).
Prerequisites: Armor Proficiency (light).
Benefit: When you wear a type of armor with which you are proficient, the armor check penalty for that armor 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 applies its armor check penalty to attack rolls and to all skill checks that involve moving, including Ride.
Special: Fighters, barbarians, paladins, clerics, druids, and bards automatically have Armor Proficiency (medium) as a bonus feat. They need not select it.
Source: Player’s Handbook (version 3.5), page 89.