This is a simple GTK tray icon to swap Bluetooth headset modes between the
a2dp_sink mode and handsfree_head_unit mode.
The A2DP Bluetooth profile has much higher audio quality, but doesn't support being used as a microphone, while the HSP/HFP profile allows a headset to act as a microphone but reduces audio quality. By clicking on the tray icon and then selecting a headset, you can swap between these at will when listening to music or using your headset for videoconferencing.
For more information, see Wikipedia: https://en.wikipedia.org/wiki/List_of_Bluetooth_profiles