A conventional humbucker (or Humbucking pickup) is a type of electric guitar pickup, first patented by Seth Lover and the Gibson company, that uses two coils, both generating string signal. Humbuckers have high output since both coils are in series and the magnetic circuit is low loss. Like a single coil pickup, a humbucker induces a slight magnetic field around the strings, which in turn induce an electrical current on the coils as the strings vibrate. The two coils have opposite polarity, opposite windings and are connected in series which causes noise and interference to be significantly reduced via common-mode rejection. Humbuckers get their name from their canceling out interference (they "buck the hum") induced by alternating current sources normally experienced with single coil pickups.
Hum is caused by interference typically created by transformers and power supplies inside various electrical equipment utilizing alternating current. As alternating current passes through a coil, it induces a magnetic field around the coil. The magnetic field may be very weak at the pickup, but once the signal is put through various pedals and amps it can become much more evident. Using a guitar without humbuckers, a musician would hear a slight but annoying hum from the amp in silent sections of the music. Sources of hum generated in the studio and on stage can include high-power amps, processors, mixers, motors, power lines, and other sources. Humbuckers dramatically reduce the hum effect compared to single coil pickups.