It's how far forward the balance point of your arrow is from the arrow's center point. Front of Center. The further forward the balance point, the higher the FOC% and the more stable your arrow is in flight.

But then, the more arrow weight you have, regardless of distribution, can cut down on arrow speed. Also, you can put too much weight up front for the arrow's spine range to handle. So you have to design your arrow to come up with the best blend of stability and speed while staying within the arrow's spine range.

It's not hard to do, just requires a little thoughtful planning.