The Bestiary cards have it, on page 2. Jagd is an area where the magic mist is too thick, making it impossible for airships and other machines to cross it. It is a place full of deformed monsters also due to the excessive mist so no one lives on the jagds. It is an area only few dare to cross on foot.

(SPOILER)Using nethicite Cid found a way to make some empire ships immune to it so some empire ships can fly over jagd.