The focal ratio is the relationship between the diameter of the main optic (aperture) and the focal length. Lets use a Skywatcher Explorer 130 for the next example, the 130 signifies the aperture of the telescope and the length of this telescope is 650 so our focal ratio would be 650/130 = 5, so our f number would be f/5.

High focal ratios are selected when there is a need for high magnification and low focal ratios give you a wider field of view. A focal ratio of f/5 would be good all purpose instrument. The image above this post was taken using a focal ratio of 20 (f/20).