The pastern is the sloping region between the fetlock above and the hoof below, made up of the long and short pastern bones. Its angle and length act as a shock absorber: too upright transmits more concussion, too long and sloping strains the tendons. Pastern conformation is a key thing buyers and judges assess.
See also: joint that sits on top of the pastern, trimming affects pastern angle
Further reading: Pastern on Wikipedia; disorders of the fetlock and pastern in the Merck Veterinary Manual.