If you are using a shield, this skill is checked automatically each time you are attacked. If successful, your attacker may miss his attack.
Raising Parrying: Parrying is not a difficulty based skill in the traditional sense. You have (skill level / 2)% chance to block an attack with a shield.
Gideon's Advice: For training of weapon skills, it is best to have two characters fight each other and bandage each other and use Anatomy on each other. Use fast speed and lowest damage weapons to increase the number of attacks per second to increase your parrying gains. GMing a fighting skill, Tactics, Anatomy, Healing, and Parrying takes little time on UOR doing this. I would recommend gathering a lot of wool and/or cotton and making it into cloth before doing this to save money. Bring a few shields as they will break from durability loss.