Skimming means to use a finishing layer of plaster on either a new plasterboard or over existing wall or ceiling to create a plan smooth finish for painting or Wall-Paper.
Skim is a flexible skimming compound to give a smooth professional finish to plastered walls. Wall Skim is more than just ready mixed crack filler; it is the ultimate and only product you would need to prepare your walls and other surfaces before you paint. Skimming is a final coat of plastering with a very smooth durable finish to it. In general a plaster skim will only be 2-3mm thick.