This is an audio connection that uses two wires. One wire carries the signal while the other wire is used as the return. This type of connection is particularly vulnerable to noise and interference, particularly with longer cable lengths.

As the two signal leads are not of equal potential difference from the reference ground signal they are considered to be ‘Unbalanced’ with respect to the ground signal.

See Cardoid.

He who makes a mistake is still our friend; He who adds to, or shortens, a melody is still our friend; But he who violates a rhythm unawares Can never be our friend.”
Ishaq Ibn Ibrahim