A punch is a device that punches a hole in the tip of the cigar.
You could use a golf tee or a pen -- I'm sure that you would not be the first. Most punches only pierce the wrapper and help to remove the cut-out piece of wrapper. Punches come in various sizes to accomodate different ring gauges.
One thing about a punch -- the open area tends to be smaller with a punch than with a cutter which may cause more tar build-up as you smoke.
You can pick up a nice punch to try out for just a few dollars.