You know how to properly wear it, how to move inside it, and how to carry yourself in combat. Rolling this skill, you can analyze defects in need of repair or recognize it's quality and origin. You'll also learn its Soak, Prevent, and other attributes.
While in combat, Armour will cancel penalties presented by your suit. A good rule is that every 3 or 4 Levels cancels one penalty; it could take more levels to cancel the really bad suit-related penalties. These penalties can arise from heaviness, limited joints, reduced vision, etc.
Example: The GM determines you can't see much from your helmet and that the full-plate hurts your swing. Your skill is level 8. The GM decides that is enough to cancel those two penalties, but you still have slowed movement from all the weight.