Retraction Prime Speed is the speed at which the material is pushed forward in the nozzle after retracting.
It is recommended to set the Retraction Prime Speed 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 unretracts 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.