This is a compositional concept to help you place the primary subject or object of your photo at the most pleasing location within the frame. When you look through your camera, imagine two equally spaced vertical and horizontal lines creating a gird of nine squares (like Tic-Tac-Toe). You want to place the point of interest at any of the four intersections of the lines. Major horizontal lines, such as the earth's horizon, or vertical lines, such as a building or tree, should be located along the rule-of-thirds lines.