While a nozzle is on stand-by, the material is kept out of the heat zone to protect it from oozing out. This setting configures how fast the material is retracted from the nozzle chamber when switching to a different extruder, and how fast it's being pushed back when switching back to this extruder.
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.
Since the retraction for the extruder switch is longer, this setting can be configured separately from the speed of ordinary retractions.