You have a watch with a screw-back. You have a screw-back opener. Most of the time you can get the back off simply by holding them in your hands. But sometimes the back is on too tightly, you need a 'holder'. This makes it easier to hold in your hands (though, really, you should put the holder in a vice).

STANDARD SIZE. Should fit all but the largest (gents) watches.
Four adjustable nylon pegs hold watch movement 23mm to 45mm diameter

JUMBO SIZE. For very large gents watches and some antique watches.
Four adjustable nylon pegs hold watch cases 30mm to 65mm diameter