Plain English Meaning

QNH is the pressure number you dial into your altimeter so it shows your height above sea level. Set it correctly and your altimeter will read the airport's elevation when you're sitting on the runway.

Full Definition

QNH is the local barometric pressure value, corrected to mean sea level, that a pilot sets in the altimeter's Kollsman window so the instrument reads true elevation above mean sea level. With QNH set, the altimeter shows the airport's published field elevation when the aircraft is on the ground, and shows altitude above sea level when in flight.