Audio Systems: International am/fm radio, wire hook, hook up

Question
On a simple four wire hook up, what does "common" mean? What does the wire do?

Answer
Common is usually the ground or common return to ground of the power supply.  It is one side of the source (power supply or battery) powering the circuit.  It is mostly the negative side of the supply but can be either.

See some of the circuit diagrams here:

http://www.ese.upenn.edu/rca/instruments/misctutorials/Ground/grd.html