Concentration is a pool of points that represent the amount of energy or attention a character has within the context of a conflict. In a straight forward fight the points represent the amount of effort the character has to shrug off a nasty blow to the shield or dodge a sword at the last second. A skilled swordsman would be able to deflect blows without much effort. Concentration points represent the times that they need to exert themselves more than is usual. Because of this when a weapon is threatening to pierce your characters defenses the damage it does is called Distraction (Dis).
A Focus of 4d6 would give you 24 Concentration while a focus of 3d10 would give 30.