What does balanced mean?

A balanced signal has two copies of the original signal. One is inverted (flipped upside down). Any interference that causes a dip in the signal causes a spike in the inverse signal. At the receiving end the inverse is flipped again. This causes all the noise in the inverted signal to now itself be inverted. So any dip in the original signal now shows up as a spike in the second copy. The two copies of the signal are added together. Everything that was supposed to be in the original signal gets doubled in amplitude. All the noise cancels out. This is an extremely effective way to "immunize" a signal from electrostatic interference while it travels.

Some gear has balanced connectors. Usually these are XLR. Be warned that the signal is not usually balanced inside the unit. Instead an op-amp is used to balance the signal right behind the connector. Typically the manufacturer does this to give the consumer the option of using a balanced cable to transfer the signal if their room happens to have a lot of interference. In this case It is not recommended to use the balanced transmission method unless noise is getting into your unbalanced lines.

An example of one manufacturer that offers both balanced and unbalanced signal paths from source to amp is Classe Audio of Canada. Using the same quality of wire for both unbalanced and balanced runs, a slight improvement in sound quality was noticed with the balanced cable.