A bow is a defect in a board that bends along the grain lines. If the board were laid across a flat surface both ends would be in the air. There is no warp across the grain. This can be caused by uneven air circulation during as the wood dries. Any bowing causes internal stresses that will make the board difficult to cut. Ripping a board that is bowed can be dangerous. Crosscutting should be done with the bow facing upwards.