Dishwasher upper dishrack roller (or wheel assembly). This roller allows the dishrack to move freely in and out of the dishwasher. The dishrack has four of these rollers. Because the dishrack rollers often wear out at the same time, you may want to replace all of the rollers together.
Roller assembly, lower dishrack
Wheel & mount assembly, upper rack track. Mounts on the side of the tank.