It is simply the amount of weight each square foot of wing surface must carry. If you had a ten pound airplane with ten square feet of wing area, the wing loading would be one pound per sq. ft. It's usually given in ounces for our models. A sailplane may have a wing loading of 10 or 12 ounces, where a scale warbird's loading may well be over 30 ounces. Just divide the weight by the wing area.