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. The theoretical ideal would be zero backlash, but in actual practice some backlash must be allowed to prevent jamming. Reasons for the presence of backlash include allowing for lubrication, manufacturing errors, deflection under load, and thermal expansion.
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.
Properly tensioned MXL belts have very little backlash and significant compliance. Curvilinear belts and belts with sharpe (60 degree) teeth may further reduce slop. As of this writing, GT2 belts are most preferred due to their intrinsic resistance to backlash.
The Z axis in most RepRap machines is screw-driven, either as a threaded rod or a leadscrew (with square, Acme, or buttress threads), and needs some mechanical clearance to operate. Backlash in a screw-driven system comes from small bits of slop between the nut and the threads. When there is backlash in the Z axis, layers will have inconsistent thickness and finished prints will exhibit banding.
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 trapped nuts with a strong spring between them, so this is a popular anti-backlash method with many anti-backlash parts posted to sites like Thingiverse. Since this increases friction in the Z axis, sometimes it may be necessary to increase the voltage to the Z motors.
- See Backlash article on Wikipedia for more insight into mechanical backlash.