In the attachment you will find the document that describes how to clean the extruder drive gear. The extruder drive gear is in direct contact with your filament, and needs to have grip on your filament to be able to move it up and down properly. 

In case your extruder drive gear has been grinding on filament during previous prints, it is possible that dirt has gone stuck in between the teeth. This dirt makes it hard for your extruder drive gear to accurately move your filament up and down during a print job as the grip is reduced.

It is therefore important that the extruder drive gear is clean