Plain English Meaning
It's the radio beam an aircraft follows down to the runway that tells the pilot whether they are too high, too low, or right on the correct descent angle for landing.
Full Definition
The vertical guidance component of an Instrument Landing System (ILS) that transmits a narrow radio beam angled upward from near the runway touchdown zone, typically at about 3 degrees, allowing a properly equipped aircraft to follow a precise descent path to the runway during an instrument approach.
Context Anchor
Seen during instrument approach training, approach , and when flying an .