This technique involves mentally dividing your image using two horizontal lines and two vertical lines, as shown in the example below. Important elements in your scene are placed along those lines, or at the points where they meet.
Consider the Rule of Thirds
Figure 3. Employ the rule of thirds by thinking about dividing the image into a tic-tac-toe board or 3×3 grid. As the image above shows, by placing the subject along one of the sides of the grid, the image is visually more engaging than photographing the subject in the very center of the grid. Image copyright © 2013 Depositphotos/michaeljung.
Figure 4. If we place the same grid on top of this image as we did in Figure 3 above, the lighthouse falls against the far right vertical line and the horizon line would fall near the lowest horizontal line. Image copyright © by Dennis Jarvis (CC BY-SA 2.0) at https://commons.wikimedia.org/wiki/File:Brier_Island_Lighthouse_(1).jpg.