A jumper is a plastic plug fitted over a pair of metal pins. Jumpers are also used to make a hard drive, CD drive, etc, either into a master or slave, depending on which pins are shorted across by a jumper plug or left unshorted. Jumper plugs change the route taken by electricity as it flows along the electrical circuits on a Motherboard. If you look closely at a motherboard, you will see small groups of metal pins, some pins will be bare, while other pins will be shorted across by a jumper plug.