Question about armor

I have recently got the Mouse Guard book. I have a decent understanding of the rules.

A person wants to add armor to there character and I understand the benefits of it. The one thing that gets me is the -1D to resist fatique. I just cant find what that refers to, as I cant seem to find it in the book.

Any Health test involving fatigue or the imposition or recovery from the Tired penalty.

Thank you, luke