Plain English Meaning
A radio signal that tells the pilot whether the aircraft is left or right of the runway centerline during an instrument approach. The cockpit indicator shows the pilot how to steer back onto the line that points straight down the runway.
Full Definition
The localizer is the lateral (left/right) guidance component of an Instrument Landing System (ILS). It transmits a directional radio signal aligned with the runway centerline, allowing a properly equipped aircraft to track that centerline precisely during an instrument approach. The localizer provides only horizontal guidance; vertical guidance, when available, comes from the glideslope.