I used to live in rural Japan and frequently visited this shrine. It's a part of the Dewa Sanzan 出羽三山 and is a major pilgrimage site in Japan. It's located in the Tohoku 東北 of Japan in Yamagata Prefecure.
The famous Haiku poet Basho frequented this place and wrote about it. This man is probably praying for purification and a safe journey at one of many shrines along the route of his trek through the sacred mountains. Each year thousands of pilgims hike these mountains wearing the simple clothes and hand-made reed sandals that have been worn for centuries. They go through purification ceremonies at stops along the way including standing under freezing waterfalls. This rite has changed very little since Basho's time.