Microphone output signals can be divided into two categories - balanced and unbalanced.
Unbalanced output transmits signals through a single conductor (and shield). This circuit can easily pick up the hum of the nearby power cord and other types of current interference, producing an audible hum, which reduces the sound quality.
Balanced output is transmitted through two conductors (and shields). The signal levels on the two conductors are the same, but the polarities are opposite (one is positive and one is negative). This circuit can also be disturbed by current, but the balanced microphone input only amplifies the difference between the two signals and rejects the same level of signal on the conductor.