Large brass bells worn while hiking in areas populated by bears or other wildlife you'd rather not experience face-to-face.
The idea is that bears and other dangerous animals almost never attack people, unless the animal is startled, or cornered. The jangling of the bells is supposed to alert bears to the presence of humans, so they will not be startled. The brochures they have at visitors' centers often suggest singing, or talking loudly, or otherwise making consistent noise that's identfiable as "human".
My great aunt gave me a nice pair of bear bells she bought at Denali National Park in Alaska. I wore them while hiking at Glacier National Park and Yellowstone. They work, too, as evidenced by the fact that I was not eaten by bears.