Plain English Meaning
A radio on the airport that lets you reach ATC or Flight Service by phone when you can't talk to them directly from the ground. You key your microphone a set number of times and it dials the call for you.
Full Definition
A Ground Communications Outlet (GCO) is an unstaffed, remotely controlled radio facility located on or near an airport that allows pilots on the ground to contact Air Traffic Control (ATC) or a Flight Service Station (FSS) by VHF radio, with the GCO automatically placing a telephone call to the appropriate facility. It is primarily used at airports without an operating control tower so pilots can pick up or close IFR flight plans, receive , or obtain and other before or .