Rotenburo 露天風呂 are outdoor hot-spring water baths. Usually, you pay an entrance that will give you access to the bathing area. An onsen 温泉 (Hot-spring) dedicated business, a hotel or a ryokan are most of the times managing the baths. But if you go to remote areas, sometimes you can find baths in the wild where you can just get naked and soak in warm water for free. Bathing in onsen waters is one of my favorite things to do in Japan, especially after a nice hike.
This is the exact google maps location (Shin hotaka yu, 新穂高の湯) (Gifu Prefecture) of the bath we found.