You fire with all your archers.
All those archers that hit then can roll to wound. You need to roll a 6 first (to wound roll), then you have to roll equal or greater than the number after the "/"(to wound roll). So basically the foe is so tough (due to armour/hide/magic/etc) it takes a miracle to wound it with the weaker strength of the weapon.
Its stated within the box that holds the Wounding Chart.
Remember, you can use Might to raise the die roll to a 6 for the initial chance to wound. That Might in turn also affects the 2nd roll to determine if you wound. Confused? Don't be, look on pg.36 ORB 5th paragraph. Even Miracles need a boost every once in a while.
-