While a nozzle is on stand-by, the material is kept out of the heat zone to protect it from degrading. This setting configures how fast the material is pushed back in the nozzle chamber after the extruder switches back.
Since the retraction for the extruder switch is longer, this setting can be configured separately from the speed of ordinary retractions. Increasing the setting can reduce oozing, but has a chance of grinding the filament which may cause the feeder to lose grip on the filament.