Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - General Explanation of Thick Skin Talent

I believe it means that your armor will be increased 5 + Man-At-Arms Level x 2. So if your Man-At-Arms is a level 1 your armor would be increased by 7 (5 + 1 x 2). If Man-At-Arms at level 2, armor would increase by 9.