Good question, Cary. True cribbing (also known as windsucking) is a stereotypic behavior in horses. Stereotypic behaviors—often called obsessive-compulsive—are characterized by repetitive movements or actions that serve no function. They can sometimes be an expression of stress or boredom.
When a horse cribs, he grabs an item, often a stall door, with his front teeth and sucks in air. Often you will hear a distinctive gulping sound as he forces air down his throat. Cribbing should be discouraged because it is dangerous in two ways: horses often swallow slivers of wood while performing the behavior, and the windsucking itself can cause colic and other issues.
Providing a horse with opportunities for equine companionship, pasture grazing and other forms of enrichment can often greatly decrease cribbing.