Sanding means polishing a rough surface with sand paper. You will need to sand when you assemble two visible parts like two half fuselages: you will want to eliminale any visible seam. Therefore you’ll have to apply putty (usually modeller’s putty or automotive filler) and sand the excess until you achieve a smooth surface. You’ll often have to putty and sand many times before you get an acceptable result.
Hope this helps!
Laurent
So many planes, so little time ...