Burnish usually means to rub something to polish, enhance or perfect. With colored pencils I am talking about taking a white or other light value pencil and going over what you already have drawn. You can use this method when you have lots of layers of fairly heavy and often more heavily applied colored pencil that you want to blend and even out. By taking the light pencil over the surface -- you can smooth the surface and blend the colors. Experiment with the color of pencil and the amount of pressure you use when burnishing.