Jotenheim is an outdoor zone, 1x3 tiles in size, along the east central coast of the world. It is adjacent to Mimer's Realm to the north, and Nidavellir to the southwest. Additionally, with the assistance of a phasing effect, one can exit to the forest to the northwest or River Gioll to the south.
The main reason to visit Jotenheim is to fight Gymir, who possesses the quest artifact Mimming. He lives in a small structure surrounded by mountains and trees, and is usually in the south region. No other trees are randomly generated in Jotenheim, so he should be easy to locate with the aid of some cartography. Since he is trapped there until the trees are destroyed, you can choose to fight him whenever you wish.