The fetlock is the joint between the cannon bone above and the pastern below, the prominent ankle on each leg. It is a high-motion, high-load joint that flexes with every stride, which makes it a common site of swelling, windpuffs, and injury. Assessing heat or filling at the fetlock is part of any routine soundness check.
See also: bone bearing weight into the fetlock, shock-absorbing segment below the fetlock
Further reading: Fetlock on Wikipedia; disorders of the fetlock and pastern in the Merck Veterinary Manual.