F numbers tell you how much light the lens allows through to the sensor or film to make an image. They work like a 2 times table, so every "stop" (or move from one F number to the next) will allow twice as much light through as the one next to it when making the aperture (light hole) bigger, and half as much light when making the aperture smaller. The tricky thing is that instead of having sensible numbers like 1,2,3,4, etc F stops have silly designations that generally run from, say, F1.4, F2, F2.8 and so on up to somewhere around F16 or F22 (depending on the type of lens). In short, the smaller the number, the more light gets in, and the bigger the number, the less light gets in.