What is an air gap in the context of magnets?
An "air gap" is a nonmagnetic material located between a magnet and an attracted object, or between two magnets that attract each other.
An air gap is best described as a break in the magnetic circuit that the magnetism has to jump across to continue a circuit between the north and south poles. Introducing an air gap weakens the magnetic hold.
An air gap can be the air itself or a solid, non magnetically conductive nonferrous material such as wood, plastic or aluminum. It can also be a layer of paint or a very uneven surface.