The 'low' is the ground return. Most radios have audio outputs referenced to chassis ground, so although the install manual will show a twisted pair for the connection, in reality you only need the 'high' connected to make it work. However, sometimes a radio will have a transformer-isolated floating audio output (King Kr86 ADF being such an example if memory serves me correctly), in which case the low side needs to go to ground and gets this at the audio panel. There is a convention is larger aircraft audio systems that calls for all audio grounds to go to a single point in order to minimise noise pick-up from the airframe. That's why you'll find headset sockets with insulated washers to keep them above aircraft ground.