"Slop!" How's that for a technical term?
The "slop" is there because the machine that's being used to make a specific part may be out of specification, is being run too fast, too slowly, or too hard, the operator isn't paying attention; it could be a host of reasons. The result is that the parts that are being built aren't always consistent in size or are undersized by intent.
When all of the slightly undersized parts are assembled into the finished item, and that finished item is put into operation, for example, a pneumatic rotary actuator , backlash will occur during every cycle and affect its operation.
That is because as every part in the rotary actuator begins moving by being impacted by the previous part that's a little under-sized, then from the time the "go signal" is given for that actuator to work, there may be an appreciable amount of time before the tooling actually moves.
Positions of the end of arm tooling will not be exact, end of arm tooling movement will result, and there will be a lag time from when the actuator piston starts to move with the ingress of compressed air, to the time the tooling starts to move.
When air is supplied to one of the air ports on the vanes or the piston inside the rotary actuator, these parts will start to move. Depending on the accuracy of the machining of the parts (and this usually correlates to the cost of the rotary actuator) the vanes and piston may move quite a bit before their motion affects the shaft. This "play" is backlash.
By the same token, when the rotary actuator has reached end of stroke, the vanes or the piston will stop before the shaft does.
Or, if an external stop is used for the tooling to get exact tooling positioning, when the tooling is stopped it will stop the shaft, but the vanes or piston inside the unit may continue to move a bit.
Depending on the complexity of the finished device, the more parts are joined together, the greater the "tolerance stack-up" and the greater the backlash to be engineered or designed out of the final usage of that piece of equipment."