Loopback ports provide the ability to send MIDI and Audio signals from within Cantabile back to an input port. ie: the signal is looped back from an output port to an input port.
By default loopback ports are disabled because they're no longer that useful. (They used to be a handy way to send MIDI signals to Cantabile's MIDI input processing where they could be picked up by MIDI bindings).
The feature still exists however and can be enabled in Options -> Advanced -> Show Loopback Ports.
When enabled, you'll notice that for every configured input port, an output Loopback port is available as a routing target.
In the above example, any MIDI sent from the media player would be routed back to the Main Keyboard input port and appear as if the events were coming from the Main Keyboard.
Care should be taken with loopback ports not to create feedback loops. Creating a circular loop of MIDI events can crash the program and create typical feedback sounds with audio loops.