A typical unbalanced audio line sends a signal from an input through a coaxial cable to an output, such as a speaker. The coaxial line is protected from outside interference by an outer covering or shield, which intercepts this interference and grounds it. However, as the circuit needs to be completed, this outer covering also carries an electric signal back to the source. It is this signal that can cause interference as it can interact with the central cable creating a loop, usually heard as a background hum.
In a balanced line, the signal is carried by two wires surrounded by a shield. The two wires each carry the signal equally, but out of phase with each other. As a balanced receiver is only set to detect out-of-phase input, it will not pick up any in-phase signal. Any interference would be transmitted by both conducting lines equally and so be in-phase and not picked up. A further advantage of balanced lines is that the shield is only grounded on one end, making it impossible to create a loop.
To convert unbalanced to balanced signal you need to use an adaptor, interface or converter, also called a balun. Depending on your needs (e.g., signal strength, sound quality, cable length), you'll find a wide range of conversion options at electronics and audio stores.