The cannon bone is the long bone between the knee or hock and the fetlock, running down the front of the lower leg. It carries weight directly and has little soft-tissue covering, leaving it exposed to splints and knocks. A flat, clean cannon is one of the marks of good lower-leg conformation.
See also: joint just below the cannon, hind-leg joint above the cannon
Further reading: Cannon bone on Wikipedia; disorders of the carpus and metacarpus in the Merck Veterinary Manual.