A caster is a wheel on a pivot that will allow what it is supporting to move in any direction. Some pianos and pieces of furniture have casters on the legs so that they can be moved more easily. A tire caster is a caster with an inflated wheel - a small tire. The mm unit refers to the diameter of the wheel, I would guess. For a picture of one, you can go to this one on Ebay: http://compare.ebay.com/like/250973548327?var=lv<yp=AllFixedPriceItemTypes&var=sbar&_lwgsi=y&cbt=y