It's the percentage of arrow weight that is forward of the physical center of the arrow. Increase tip weight, and you will increase the percentage of weight that is forward of the arrow's middle. High FOCs tend to keep an arrow on the same flight path when there is an outside force acting on it in a direction different from it's intended travel. For instance, a side wind against a broadhead, nicking a twig on a shot in the woods, hiitting grass blades in a field, etc. High FOCs are beneficial in these situations.