The guide numbers on external flashes are based on a certain equation. The said equation states that the light that the flash produces is equal to the distance of the device from your subject. Then, it is multiplied by the lens' aperture.  For a standardized and straightforward process, manufacturers use ISO 100 and a 10-inch distance as a fixed reference point when calculating the flash's guide number.  For example, if your flash has a 60 guide number and the aperture of the lens is f/4, and the fixed point is 10 inches, then the equation will be 10 inches multiplied by f/4 equals to 60. That means that the light of your flash can cover subjects up to 60 feet.