It simply means cutting an angle so the aileron can move. Look at the photo on this page that shows the piece going through the table saw. YOu don't need the saw. A sanding block can accomplish the same thing, but the photo shows what the bevel looks like. This piece is double beveled which is probably what you need to do. Some planes have a single bevel, but it's not too common.

http://airfieldmodels.com/informatio..._points/03.htm