Gloss lamination gives a shiny, high-gloss finish to your folders, and provides extra protection against scratches etc. It cannot be written on with a ballpoint pen, so any areas which need to be written on should be left unlaminated.
Gloss laminating to one side, using silk stock, allows you to have a shiny finish on the outside of your folder, whilst leaving the inside with a silk finish.
See also: Matt Laminated