Definition
121.121 refers to the radio frequency 121.121 MHz, used in the very high frequency (VHF) band for aeronautical communications. In the context of arrival navigation, frequencies in this range are assigned to specific air traffic control (ATC) facilities, flight service stations, or other aeronautical services that pilots tune to for communication during a particular phase of flight.
Plain English
It is a specific radio channel — 121.121 megahertz — that a pilot tunes their VHF radio to in order to talk to a specific air traffic service.
Context Anchor
Seen in FAA handbook figures or instrument chart examples where small printed numbers, frequencies, and chart labels appear close together.
Derivation
The number is simply the frequency in megahertz (MHz). Aviation VHF voice communications use the band from 118.000 to 136.975 MHz, and each facility is assigned a specific frequency within that range so transmissions don't overlap.
Why Pilots Care
Failure to comply can result in regulatory violations and loss of situational awareness during arrival.
Intuition Check
Do not read the dash as a minus sign, and do not treat it as part of what you tune. First confirm what the number is labeling on the chart, then use the published value exactly.
Example Sentence 1
Approaching the arrival fix, the pilot tuned 121.121 on the COM radio and checked in with the controller.
Example Sentence 2
Dispatch confirmed the aircraft met 121.121 standards for the destination airport's approach plates.