Backlash

Backlash is an effect caused by "slop" (too much freedom of movement, or too little constraint) in a mechanical drive system, leading to small discrepancies in positioning at points where axes reverse direction. The simplest case is in a pair of spur gears. Backlash is the amount of clearance between mated gear teeth. When the direction reverses the drive gear has to move past this dead zone before there is any axis movement.

Belt Drives

Pulley backlash is not like gear backlash. You don't get a dead spot when you change direction because the teeth don't slip. Instead, when you change direction the belt teeth (accidentally) engage at the opposite side of the valley. After half a turn in the new direction all the teeth will be engaged on the new side, and the distance travelled will be a little short. So where it stops depends on which direction the axis came from and also how far it has travelled in that direction. (i.e., The backlash is distributed over a full half turn rather than happening immediately on direction change.) This can occur from belts that are too loose or too tight, so some finesse is needed to properly tension drive belts.

Z Axis

The Z axis in most RepRap machines is a screw drive, and needs some mechanical clearance to operate. One way to minimize backlash in a screw-driven axis is to use a tight tolerance Delrin nut. A less expensive solution is to use two nuts with a spring between them, and so this is a popular anti-backlash method.