See also: Feats
You are proficient with Medium Armor, and can wear it without impediment.
Prerequisite: Armor Proficiency (Light) Feat
Normal: A character who wears Medium Armor with which they are not proficient takes a -5 Armor Check Penalty on attack rolls as well as Skill Checks made using the following Skills: Acrobatics, Climb, Endurance, Initiative, Jump, Stealth, and Swim. Additionally, the character gains none of the armor's special Equipment bonuses.