What's meant by F.O.C.in archery?

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FOC is "front of center", which is a term for how far the center of mass is relatively from the midpoint of the arrow. There doesn't seem to be a consensus on what an "ideal" FOC is, but I hear anything in the 7-15% range will provide a good balance of distance versus stability (source). It can still be used as a way to measure the weighting/balancing/feel of an arrow (e.g. how front-heavy it is).

To calculate it, install the tip and find the center of mass by sliding the arrow along a narrow edge until it balances. Find the distance from here to the halfway point, and divide by the total length to find the FOC. In the image below, the FOC would be 10% because 3 / 30 = 0.1.

_{(source: goldtip.com)}