It's the ratio of viscous to inertial forces on an aerodynamic body.
On a small body, the inertia of the air will be more significant than on a large body. Conversely, a large body will be affected more by viscous forces (i.e. skin drag) than a small body.
Calculating Reynolds number gives a means of adjusting scale data to give some sort of meaningful result from scale testing. Another way around it is to use a different gas in a scale test to give the same Reynolds number as the full-scale body.
Have I confused anyone??