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What this means is that you have got to great a space between successive gear teeth.
This is usually the result of wear that occurs to the teeth of the gears and although they must have a small gap to allow them to be correctly engaged when adequately lubricated, to great a gap is generally what excessive back lash alludes to.
This wear will mean that your gears will not mesh correctly, which in turn changes the position of the driven cylinder in relation to the drive cylinder.
This generally leads to mis-registration and no amount of adjustment will correct it until you replace the gears that are worn.
If you have multiple gears in a chain then this problem will simply be multiplied and will make the registration issue even worse.
Backlash can and will cause vibration on the machine as well; this normally shows in the amount of plate bounce you may experience and will certainly add to even more print issues.
Bottom line excessive spacing between the teeth of your gears as they wear will make the achievement of good quality print impossible. As a result some gear trains are built with anti-backlash gears that allow on press adjustment without the need to immediately replace the worn gears immediately.