It refers to the type of cables used. Cables without a ground wire are likely to add noise to the output.
"An unbalanced cable consists of two connectors with two conductors each, connected by two wires inside the cable—a signal wire and a ground wire. You can quickly (in most cases) identify a cable designed to carry an unbalanced signal by its connectors: because each wire has to terminate at the connector with its own contact point, an unbalanced cable requires only two conductors at the connector. A standard TS (or “tip-sleeve”) guitar cable is the unbalanced cable you’ll run into on stage most. Standard RCA cables used for many AV components are also unbalanced cables."