Plain English Meaning

When the aircraft is flying level toward the runway and crosses the descending glideslope beam, the autopilot/flight director switches itself over and starts following that beam down for the approach.

Full Definition

A flight director and autopilot mode that automatically engages when the aircraft intercepts the ILS glideslope from below, transitioning the system from altitude hold (or whatever vertical mode was active) to tracking the glideslope signal down to the runway.

Context Anchor

Seen on the flight director or autopilot mode display during an Instrument Landing System approach, usually as the aircraft reaches the glide path from below.