Silverskin is a shiny silver connective tissue that runs along the back of the pork tenderloin.  It is tough and fibrous so you will definitely want to remove it.   To remove the silverskin, slide a sharp knife between the silverskin and the meat and slowly slide your knife along the silver skin until you have removed it.  Try not to cut too deep into the meat.  This guide with step-by-step instructions from Ontario Pork will help you.