A quote from the Official Manual-
Shielding is the ability to successfully block enemy attacks with a shield. Of course, characters need to hold shields in their hands in order to use this skill. Note that it is not possible to block more than 2 opponents at a time.
So, if you use a bow or two-handed weapon your shielding skill won't be used because you need to be wearing a shield. I just wanted to add one more interesting fact about shields in general from the Official Manual-
Items determine a character's performance in combat in accordance to their nature. For instance, armor such as legs or helmets will reduce the damage caused by successful hits, while the bonus yielded by shields is used to calculate a character's chance to block attacks. Special clothing and certain amulets can also help you to reduce the damage of a certain type. Furthermore, you should choose your weapon, runes and spells according to the characteristics of your opponent.