The difference in the two frequencies is fed into a modified form of the above equation and a speed is calculated. We dont just have the word of Maxwell and Einstein for what the speed of light is though. All this means is that the velocity of an object is moving faster than light from one single point of reference.
This was realized by Maxwell circa 1864 as soon as the equation c 1e0m012 2998 X 108mswas discovered since the speed of light had been accurately measured by then and its agreement.
C f where. Another way of writing this is. Light is an electromagnetic wave. All you really need to know is that it acts as a conversion factor between mass and energy.