This constant represents the volume of a standard cord of wood.  You may select other than cubic feet as the unit of volume.

Definition of a Cord

A cord of wood is a standard measure of firewood which is defined to be a tightly stacked pile of wood that is 4 ft. by 4 ft by 8ft -- or 128 cubic feet.

To make the measurement of the stack of wood easier, a cord is usually stacked between straight uprights separated by eight feet, as shown in the picture below, making the calculation a straightforward rectangular volume.