Reynolds #s
A Reynolds # describes the relations between interial forces and viscous forces. Thes # are used to descrbe any flow with relation to an object. With airfoils we are concerned with the size of an airfoil the velocity of the airfoil and viscosity of the medium in which it travels.
Think of a reynolds # as an efficiency factor. Since we really can't change the medium we fly in, we really only need to be concerned with airfoil size and velocity. This is not exactly correct in that our medium (air) does change with temp, humidity, altitude, etc.
As to velocity and size, fast speeds and big wings are more efficient. Because the medium (air) is still the same viscosity for both, the size and speed matter. Depending upon how fast something moves and how big it is the air "feels" different to the airfoil. Again not entirely exact, but layman's terms.