What they are talking about it using covering material (monokote or ultracoat) to seal the gap so that air cannot escape through the gap. (see first diagram) IMHO this is probably the one best thing you can do to improve perfomance on any plane. It's not difficult to do at all. First cut a strip of covering material and fold it in half. A small trim iron is best suited for this but a regular covering iron will work, it's just a bit more cumbersome. Use a piece or two of masking tape to hold the aileron at full deflection so that the gap is wide open. Now use a trim iron to seal down the covering to one side of the gap. Then do the other side. see diagram

Hope this helps

Ken