The Weapon Bonus is a modifier of the Attack and Parry values. The first value is added to the effective Attack value, the second is added to the effective Parry value. For example due to the speed a Foil (Weapon Bonus 1/-1) makes it a little easier to hit the opponent (a bonus of 1 to the Attack value), but as it's a very light weapon it makes parrying harder (Decrease of 1 on the Parry value). A Wooden shield (Weapon Bonus -1/3) will restrict the fighter slightly when striking (a Decrease of 1 on the Attack value), but instead it will noticeably improve the Parry (bonus of 3).