It is a fudge factor to allow models to be used in wind tunnels for aerodynamic research. The idea being by factoring your results by the RN, you get the aerodynamic characteristics of the full sized aircraft.
It shouldn't be of too much interest to pilots, but unfortunately some aerodynamics books have got a bit hung up on them, probably because the author is on a mission to prove how much more they know than the reader IMHO.