Some weapons require a minimum level in a specific characteristic to be effectively used. For example, a Heavy Bow might require a minimum of +2 in Strength (STR).
If this condition is not met, the weapon can still be used, but a penalty equal to the difference in points between the current score of the characteristic and the required threshold is applied to the attack roll.
Example:
A character with a Strength (STR) of +1 wants to use a Heavy Bow that requires a minimum of +2 in Strength. The difference is 1 point (+2 required - +1 current). Therefore, the character will suffer a -1 penalty to their attack rolls when using this Heavy Bow until their Strength reaches or exceeds the threshold of +2.