In simple terms, airplanes have a device called a transponder. It sends out a 4 digit code. You can set the digits from the plane and change them. It is generally used to identify a certain flight. This code may be assigned by air traffic control, or you may set it to a general code, or you can turn it off. What you " squawk" is your transponder code. Certain codes are reserved for certain situations such as general emergencies, hijacking, lost comms, etc.