Reynolds number (Re) is basically the most important parameter for aircraft design, at least when you're traveling slower then Mach 0.7 or so. Above that, then both Reynolds and Mach numbers are the most important. You don't really use it directly in formulas like you do Cl or Cd, it's more of like, Re kinda dictates what you can expect the airflow to do over a body. I'm really not sure how to explain it, try reading a beginning text on aerodynamics. Introduction to Flight by John D Anderson is a good place to start. There was also a big thread on Reynolds number here a while back, that might explain things better.