may be a dumb question, but its a thin piece of clear tough sticker that seems to be placed over the resistors i believe when its screwed on. never seen that before
it serves 2 purposes. 1 to help dissapate some heat and 2 to keep the cover from coming in contact with it. sometimes the cover are very thin and if you had the amp on a uneven surface then the cover wil bow in and short out.