Airbuckers are full-sized humbuckers that contain a gap or space between the magnet and the polepieces. The distance is quite small, and may be filled with nothing but air or by some non-magnetic material. The purpose of the gap is to lessen the magnetic field over the pickup, which in turn lets the string vibrate more freely. This results in a purer, more open sound and improved sustain.
The other purpose of the “air” design is to reliably reproduce the performance of vintage humbuckers (1950s and 60s), many of which contain magnets weakened by age and mishandling. Weakened or "aged" magnets are inherently unstable, and therefore cause unpredictable performance. Airbuckers utilize stable, full-strength magnets, yet the magnetic field is the same as produced by an "aged" magnet, thus offering the performance advantage without the unpredictability.