Retraction Speed

This is the speed at which the material is pulled out of the nozzle and pushed back in, when making a retraction.

In general you'll want to set this as high as possible without grinding the material at the feeder. Moving the filament at greater speed makes the nozzle stand still for a shorter amount of time, which reduces the size of the blob in the location where it moves the material and slightly reduces printing time.

If the speed is set too high however, the force exerted by the feeder will be so high that the feeder wheel will start to grind into the filament. This reduces the reliability of the print.