If you're talking about a camera then these numbers refer to the maximum size of the aperture of the lens. The larger the aperture, the more light that can pass through the lens. A larger aperture means a better ability for a camera to cope with low-light situations and enables faster shutter speeds to be used. The advantages of a faster shutter speed are being able to freeze action easier for moving subjects and also to reduce the chance of camera shake which can result in a blurred image.
