means that basically the amount of movement (distance, speed, etc) of the surface (or servo) are the same as the movement of the stick on the radio.....
ie: move the aileron control stick 1/2 way...the aileron only goes 1/2 way...
move it fast... the surface goes fast, move it slowly the surface goes slowly

I'm sure there is a much better description of it somewhere... but that is how I describe it....