Why, the mark of the beast, of course!  HA, I KID.  The UPC/EAN is a numerical code, usually associated with a barcode, that uniquely identifies your product for sale.  You need to buy one, usually, for a nominal fee.  Sometimes your label has ’em.  Sometimes you can get them from your distributor (Tunecore and ReverbNation will allow you to get a UPC for your release).  If you want to sell your music in stores, physical or virtual, you need a UPC.  The UPC code also can (and should) be embedded in the CD metadata itself.