| A "humbucking" pickup is a type of pickup constructed in such a way the externally induced hum is eliminated. A normal pickup consists of a magnet (or
several magnets) wrapped in a coil of wire. A steel
guitar string, vibrating above the pickup induces a current
in the wire that corresponds in a very direct way with the
vibration of the string. However, the pickup is subject to
having current induced in the coil by other things besides the string. House electical circuits often induce a 60 Hz hum, for example.
The humbucking pickup works by actually having two pickups. A current is induced in both pickups by the
vibrating string. However, the pickups are arranged such
that the current induced in one pickup is the mirror image, the negative of the other. However, signals induced from other things, RF noise, 60 Hz AC circuits, etc. are
not mirror images, are not reversed.
Then, the two signals are subtracted one from the other. This has the effect of doubling the strength of the induced signal from the vibrating guitar string while nearly eliminating the noise component from the signal.
So the troublesome "hum" so common with "single-coil pickups", is cancelled. | 
