Plain English Meaning
A radio-to-phone box at an airport that lets a pilot on the ground talk to Air Traffic Control or Flight Service when there is no tower nearby. The pilot keys the radio a set number of times, and the GCO automatically dials ATC or Flight Service over a phone line.
Full Definition
An unstaffed, remotely controlled ground-to-ground communications facility installed at airports without an operating control tower or after tower hours. Pilots use the GCO on the ground to contact the nearest Air Traffic Control facility or Flight Service Station via VHF radio, which the GCO then connects through a telephone link. It is intended primarily for activating or closing and obtaining or while on the ground.