It’s a dot sight, often similar to a red-dot sight used on rifles, adapted for camera use. Wildlife photographers use them to help quickly find and track small or fast-moving subjects when a long telephoto lens gives a very narrow field of view. Instead of trying to locate the subject through the lens first, you keep both eyes open and use the dot sight to aim the camera more easily. There are also camera-specific versions made for this purpose, such as the Olympus EE-1 style of sight.