You are proficient with heavy armour (see Table: Armour and Shields).
Prerequisites: Armour Proficiency (Light), Armour Proficiency (Medium).
Benefit: When you wear a type of armour with which you are proficient, the armour check penalty for that armour applies only on Balance, Climb, Escape Artist, Hide, Jump, Move Silently, Sleight of Hand and Tumble checks. Double the normal penalty applies on Swim checks.
Normal: A character who is wearing armour with which she is not proficient applies its armour check penalty on attack rolls and on all Strength-based or Dexterity-based ability and skill checks.
Special: Fighters, Paladins and Clerics automatically have Armour Proficiency (Heavy) as a bonus feat. They need not select it.
Source: Player’s Handbook (Page 89)