Imagine lines drawn across the image dividing it into thirds both horizontally and vertically. You place important subjects of your composition where these lines intersect or position elements of your picture on the lines themselves.

 

Good examples of this approach are the horizon placed on either of the two horizontal lines, or a tree or person positioned on the vertical lines.

 

Using the Rule of Thirds helps produce nicely balanced pictures, avoiding the generally held principle that objects or subjects should not be placed bang in the middle of the frame, I'll let you decide as in most rules they are there to be broken.

 

See how many pictures in my gallery abide by the rule and how many have broken it.!