its the amount in centimeters that the binding is mounter either FORWARDS or BACKWARDS of the center mark (the mark on the ski denoted by a manufacturer as to where the center of the sole of your BOOT should be)
easy recomendation, unless its a SHORT ski, mount it regular, especially like u said, your new to this, so go with what the manufacturer says
if the skis short, u might want to mount it 1-2 cm forward, to get more tail in your skiing, thats what we did on a pair of 160 armadas at the shop i work at.
just ask the guys where your getting the skis and bindings mounted, theil set you up right