Normally used with the Crstyal Mic to plug in your music player or phone, this adapter can also be used  to plug in your music headphones into the aircraft intercom. 
 
If you use a 3 pole plug (TRS: left audio, right audio, ground) then you will just hear the audio. If you use ordinary headphones without a microphone you can use the aircraft hand held microphone (if available) to speak.