Reynolds numbers are a reference for describing the viscosity of air, relative to our models. Ollie or someone can give a more precise explanation, but it basically boils down to this:

We can make our airplanes any scale we want, but air molecules don't oblige by changing their size. At a model size, the air molecules are effectively "larger".

So...
What is true, in terms of airfoil performance on a full-scale plane is not true for a model-size airfoil. Andy Lennon's book "The Basics of Model Aircraft Design" has some good information that will help you apply this fact. (Model Airplane News has the book.) When you choose an airfoil for a model, it's good to know how it performs at different Reynolds numbers. I don't try and memorize these things... Just look them up when I need them.