This refers to the lines found in wood that form its signature texture.
That texture is created from the cutting process in making wood planks; lumber is cut for length of the tree that creates that pattern. If the tree was cut horizontally, you would see the wood rings instead. The saw process also determines the look of the grain. A plain sawn wood produces a grain where some of the grain looks like a frame. A quarter-sawn plank of wood produces more straight, longitudinal grains.