The f-number or focal ratio is just the focal length of the spotting scope divided by the aperture (diameter of the lens). It’s almost universally given as a quantity like f/5, f/2.8, etc., called the f-stop or photographic speed. These terms are used interchangeably.
Example: an 80mm spotting scope with a 480mm focal length has a f-number of 6 and its photographic speed is f/6.
Lower numbers mean the lens is delivering light more quickly and consequently shorter exposures can be used when taking a picture through the scope.