The difference between F5.6 and F11 is in a nutshell the size of the opening in the aperture to allow the amount of light into the camera and onto the film. The F11 is four times smaller than F5.6. Each step is 1/2 of the previous.
F1.4, F2, F2.8, F4, F5.6, F8, F11, F16, F32, F45, etc......
So each step up (1.8 being the biggest opening) the Iris closes by 1/2.
The difference in exposure for that same image with the same light source the image with the F11 would be 4 times darker than the image with the 5.6.
