Forward of Center is a measure of how far the balance point of the arrow is in front of the center point of the arrow. The more weight you put up front the farther that fulcrum point moves toward the tip and the higher your FOC becomes. It's especially important in controlling arrow flight with broadheads. A low FOC gives the broadhead more control over the flight of the arrow as opposed to the fletching having more control on a higher FOC shaft. I've had decent flight out of shafts down as far as 11% FOC but prefer my current shafts at 15%. Another benefit is that a high FOC shaft has the fulcrum point farther forward around which it flexes on impact thereby decreasing the amplitude of deflection and allowing the shaft to spend that energy on penetration rather than deflection.