Humbucking pickups or humbuckers are a type of electric guitar magnetic pickup. A typical humbucker consists of two coils of wire wrapped around a set of polepieces and a single magnet mounted to a single baseplate.
Humbuckers use two out-of-phase coils situated side-by-side which cancel out the interference picked up by each coil. Particularly, humbuckers eliminate (or significantly reduce) interference introduced by the alternating current power supply, so called 60-cycle hum or mains hum.
Humbuckers typically have adjustable pole pieces (typically screws) on one coil and non-adjustable "slugs" on the other coil. That said many variations exist which use "staple" pole pieces, bars, or other arrangements.
The other type of magnetic pickup commonly found in electric guitars is the single coil pickup which, as its name implies, has only a single coil and is thus more susceptible to mains hum.