A lean-to is a free-standing structure of three walls with a sloping roof, usually used for camping, and located near a lake or pond, or in the mountains. The open side is sheltered away from the prevailing winds and rains. It is usually a rough structure made of logs or unfinished wood.
See guide Joe Hackett's Adirondack Gadabout column in the Adirondack Daily Enterprise of December 8, 2012, for more on lean-tos.