Also known as a "contrast viewing filter" this is a round piece of darkened glass which is held in front of the eye to evaluate the overall contrast of a scene. Our eyes are very good at handling a large range of brightness from the darkest shadows to a bright sky, but film or video cameras react differently and can only resolve detail within a much narrower range of brightness before becoming under or over exposed. Looking through the pan glass approximates what the camera will see and can help the photographer to set an exposure or to decide whether to add light to the shadow areas of the scene.
