Attack Combat Value (ACV) = (B + M + S) / 3. After also adding in successful, relevant skills & attributes to your ACV, when attacking you must roll your total ACV or lower in order to land a successful attack. (Always round down for original ACV.) i.e. Your original ACV is 6, and you have Level 2 in Unarmed Attack, thus upping your ACV from 6 to 8 when you perform an unarmed attack only.
If you roll a 2, regardless of any modifiers, this means a critical success. And of course, if you roll a 12, this means a critical failure.