It is another term used for a “pickup.” It converts the mechanical vibration of a string when plucked through a magnetic field within a pickup. As the moving magnetic field moves back and forth through the coil, an alternating current or signal is produced. As the signal travels through a volume and tone control or other circuit if used the signal travels to the jack. Connecting a shielded cable between the instrument and amplifier allows the signal from the instrument to be amplified and heard through the speakers.