An air caster is a pneumatic lifting device used to move heavy loads on flat, non-porous surfaces. Its operation is similar to a hovercraft

Hovercraft

A hovercraft is a craft capable of traveling over surfaces while supported by a cushion of slow moving, high-pressure air which is ejected against the surface below and contained within a "skirt." Although supported by air, a hovercraft is not considered an aircraft.Hovercraft are used throughout...

, as it uses a thin layer of air as a way to float a very small distance off the ground. Compressed air enters an airbag

Airbag

An Airbag is a vehicle safety device. It is an occupant restraint consisting of a flexible envelope designed to inflate rapidly during an automobile collision, to prevent occupants from striking interior objects such as the steering wheel or a window...

 shaped like a torus

Torus

In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...

, and when the bag is filled it creates an airtight seal with the ground, and forces more air into the centre of torus, eventually causing the air to flow over the bag and to raise the load above the ground.

The compressed air is forced under the airbag, pushing it and the load less than a millimetre off the ground.
Because air casters are virtually friction-free, the force required to move them is very low, often around one to five lbs.