Plain English Meaning
It is how high the airport sits above sea level, measured to the highest point of its runways.
Full Definition
The published height, in feet above mean sea level (MSL), of the highest point on any of an airport's usable runways. It appears on instrument approach charts as a reference for pilots planning the approach and landing.
Context Anchor
Seen on instrument approach procedure charts, usually in the airport information area or near the airport sketch.
Derivation
“Apt” is a shortened chart form of “airport.” “” comes from a word meaning “to raise up,” which the use: it tells how high a place is compared with sea level.