Plain English Meaning
A radio that can tune to extra, more closely-packed channels so controllers have more frequencies to hand out. Without it, the pilot simply cannot select some of the frequencies in use today.
Full Definition
A Very High Frequency (VHF) communication radio capable of tuning to channels separated by 8.33 kilohertz, rather than the older 25 kHz spacing. The narrower spacing triples the number of available frequencies in the aviation VHF band, easing congestion in busy airspace. This capability is required for operations in much of European airspace and may be specified by certain procedures or air traffic control assignments.
Context Anchor
Seen in aircraft equipment listings and flight planning information when identifying what the has available.