The collar of slightly longer grass ringing the putting green — cut shorter than fairway but not as tight as the green itself. It matters because it changes your options: from the fringe you can putt (the ball just rolls a touch slower through it), chip, or use a hybrid like a putter. Many golfers putt from the fringe whenever they can — the old saying "your worst putt is usually better than your worst chip" exists for a reason. Note: fringe is not the green, so you can't mark and clean your ball there during normal play.