When you have a Full Length Re-Sizing Die screwed all the way into the press so it makes contact with the shell holder, the die full-length re-sizes the case.  This means it returns it to factory dimensions.  If you are reloading ammunition with cartridge cases that were fired in your rifle, the brass case has expanded to close to the dimensions of your rifle's chamber.  It is unnecessary to fully resize the case and needlessly works and stretches the brass case.  Backing-off the Die, a turn or two from being fully threaded into the press, allows the Die to just resize most of the case neck and not touch the rest of the case.  This practice has accuracy advantages and makes for longer case life (more reloadings).  It is worth while to get a reloading manual and read the begining chapters on reloading basics before investing in equipment.