Meaning: The first number of Strength Bonus (12) indicates the minimum ST that is required to properly use the weapon. The second number (3) dictates the strength increments at which damage increases by 1.
Rhulana, who has ST 15, will do 1D6+5 Damage with this weapon. At ST 18, she will do 1D6 +6 Damage with it. ST 16 and 17 has no effect on weapon damage.