Plain English Meaning
It is the shared radio channel pilots use at airports that have no controller on duty. Everyone in the area listens and announces what they are doing so they can stay clear of each other.
Full Definition
A designated radio frequency used at airports without an operating control tower (or when the tower is closed) on which pilots transmit position and intention reports to coordinate traffic with other pilots in the area. The CTAF for a given airport is published on charts and in the Chart Supplement and may be a UNICOM, MULTICOM, FSS, or tower frequency depending on the airport.