The hock is the large, angular joint midway down a horse hind leg, the equivalent of the human ankle. It is a primary source of hind-end power and a frequent site of arthritis (bone spavin) and swelling; heat, filling, or stiffness in the hock is a common reason for hind-limb lameness.
See also: bone running down from the hock, high-load joint below the hock
Further reading: Hock (equine anatomy) on Wikipedia; disorders of the tarsus in the Merck Veterinary Manual.