An F stop is basically the size of the aperature that allows the light into the camera, or how much light that is able to get in. Basically the smaller the number, the faster the lens is or the lower the amount of light you need to take a picture.(the bigger the hole, the more light that comes in the LOWER the F stop. Sometimes you can play with your camera and allow more light in than needed to overexpose a shot or vice versa.