In relation to R/C helicopters (that are mostly of the two blade variety) disc loading generally refers to the load (weight) being carried relative to the size (area) of the rotor disc.

e.g. A 60 size heli with a rotor head using 710mm blades and weighing 11 lbs. will have a lighter disc loading then if it used 680mm mainblades carrying the same 11 lbs. Everything else being equal (using the same airfoil blades design etc. and starting with the same head speed and into the same headwind), the lighter loaded disc (710mm) will have a lot more reserve in the auto.