Plain English Meaning
The slower of two speeds the engine's air-compressor can spin at. It is used down low, where the air is already thick enough that you don't need the compressor working at full speed.
Full Definition
On a two-speed supercharged engine, the lower of the two impeller gear ratios, used at lower altitudes where the air is denser and less compression is needed to maintain rated manifold pressure.
Context Anchor
Seen in discussions of supercharged piston engines, especially when managing engine power as the aircraft climbs or descends.
Derivation
Blower' is an old nickname for a — the device that 'blows' extra air into the engine. 'Low' refers to the lower gear that drives it, not to low .