FOC stands for Front of Center. You care, mainly, because of fixed blade broadhead flight. It's basically physics. The vanes use leverage based upon where the balance point is.

The "experts" say that you should maintain between 9 and 12% front of center balance point. Some others say 10 to 15, so practically, I try to make it to 10%.

Go to www.bowjackson.com and look under FOC. There's a calculator and some pictures to show what to measure. I've found that the calculators can be misleading, so you probably want to measure yourself. The formula is FOC = 100 * (Balance point)/(arrow mid-point) where the balance point is the distance from the arrow mid-point to the actual balance point, versus from the nock.

Hope this helps. Do a search for FOC and you'll find many threads discussing it!