Plain English Meaning
A radio signal that tells you whether you are lined up with the runway, or off to one side. It shows you how far left or right of the runway centerline you are during an instrument approach.
Full Definition
The component of an Instrument Landing System (ILS) that provides lateral guidance to the runway centerline. The localizer transmits a directional radio signal that an aircraft's navigation receiver interprets as left/right deviation from the extended centerline of the landing runway.
Context Anchor
You will see localizer information on instrument approach charts and on navigation displays during an instrument landing system or a localizer-only approach.