Plain English Meaning
A supercharger with two gear settings. The pilot picks a slower setting for low altitudes and a faster setting for high altitudes, so the engine can keep getting the air pressure it needs as the air outside gets thinner.
Full Definition
A supercharger fitted with a two-speed gear drive that allows the pilot to select either a low-blower or high-blower setting, changing the speed at which the impeller is driven by the engine. The low setting is used for takeoff and lower altitudes; the high setting is engaged at higher altitudes where the thinner air requires the impeller to spin faster to maintain manifold pressure.