The ability to deflect attacks with a shield
Parrying is a common choice of the warrior, both for its physical protection and for its passive protection.
All that is needed to parry is to equip a shield, and enter combat. As your skill rises, the AR bonus you receive from your shield will rise.
Unlike most skills on UO: Invictus, training with a NPC prior to beginning your path would most assuredly be a waste of gold. The simplest means of training this skill is to engage in melee combat while equipped with a shield. A common choices would be the heater shield, for its superior AR.
A common method of macroing this skill is to spar with a friend while cross healing each other with band aids. Your skill will rise as you are stuck in combat. Most choose this method while using some of the lowest damaging weapons such as a dagger or knife.
Player vs. Player
There is a passive bonus to those who wear shields. Parry will reduce explosion potion damage by 1 point per 20 points in parry skill. At GM parrying, you will have a 20% chance to reduce explosion potion damage by up to 50%.