A standard audio connection (RCA plugs and jacks) are considered 'unbalanced", because the two conductors, the 'hot' and the shield, are at different voltage references; the shield is grounded, and the center conductor is not.
A balanced interconnect contains two ungrounded conductors, both insulated, and a separate shield. The desired signal is a 'difference' signal, which is the voltage between the two insulated conductors.
The advantage is that, if fed to a "true" unbalanced input, with what's known as a 'differential amp', any electrical interference picked up along the way is rejected, because it's of like polarity (common mode) on both wires.
Only the difference signal is amplified; the common-mode noise is ignored by the circuitry's "common-mode noise rejection" capability. This is also used by such transmission systems as CAT-5 (twisted-pair) networks.
The twisting assures that the noise is induced equally in both conductors. The higher the twist-per-foot count, the higher the frequency of noise that can be rejected, because higher frequencies have shorter wavelengths.
Only a noisy environment really makes for benefits from balanced lines, and with longer runs. The normal A/V rack has 1- or 2-m shielded cables, which are adequate unless the cables are very crappy (
