It's the unit of measurement for hardwood lumber when it's sold in random widths and random lengths. It's a volume measurement that's equal to 144 cubiic inches.
We have a more comprehensive explanation here.
Wood that's cut to specific sizes will be sold by the piece or sometimes by the lineal foot. Example would be turning squares and 1x6 hobby boards.
Wood that is thinner than 1" thick is sold by the square foot.