They are known as the Sel-Cal codes (short for SELective CALling).
When flying a long distance, so that the pilots don't have to keep a listening watch all the time, ATC inputs the four letter code into a machine so that it "chimes" inside the cockpit of the aircraft concerned to alert the pilots to get in touch with ATC.
The same code can be in existence for 2 or 3 different aircraft so they tend to make sure that all the planes do not operate in the same area in case the wrong plane answers.
The codes are designated by an independent body (which I can't think of the name at the moment...Airinc?)
David/MAN: 258 days and counting to the next Commonwealth Games