Plain English Meaning
The radio channel pilots use at a small airport without a control tower to talk to the people on the ground and find out things like which runway is being used and what the weather is doing.
Full Definition
The radio frequency assigned to an aeronautical advisory station (a UNICOM) at a non-towered airport, used by pilots and the airport operator to exchange airport advisory information such as wind, runway in use, traffic, and field conditions.
Context Anchor
Seen in the Chart Supplement U.S. airport entry when a is checking which to use for local airport advisory information.