Plain English Meaning
It's the shared radio channel pilots use to talk to each other near small or untowered airports. Instead of a controller telling everyone what to do, pilots announce what they're doing so other pilots in the area know who's where.
Full Definition
A radio frequency designated for pilots to broadcast their position and intentions when operating at airports without an active control tower, so that nearby aircraft can coordinate with each other directly. The frequency may be a UNICOM, MULTICOM, FSS, or tower frequency (when the tower is closed), and is published on aeronautical charts and in the Chart Supplement.
Context Anchor
Seen on airport information pages, , and in for airports that do not have an at that time.