Plain English Meaning
It is the box on the approach chart with a thick black outline. The thick outline is a visual signal telling the pilot, 'this is the main navigation aid for this approach.'
Full Definition
On an instrument approach chart plan view, a NAVAID information box drawn with a thick (heavy) border, indicating that the navigation aid inside the box is the primary facility used for the published approach procedure. NAVAIDs shown in thin-lined boxes on the same chart are secondary or supporting facilities.
Context Anchor
Seen in the plan view of FAA instrument approach charts, near the symbol for the navigation facility.