A board foot is the standard unit of measure for lumber in the US. It is a measure of the volume of wood (length x width x thickness) it is calculated by multiplying the square footage of one face of the board by the thickness in inches (see next section for conversions).
The formula to calculate board footage is (all measurements in inches)
(Length x width x thickness) / 144 = Board footage
Thus a 6” wide 4’ long piece of 4/4 lumber is equal to 2BF