The Rule of Thirds divides the viewfinder in both 1/3rd horizontals and verticals. A dominant feature is placed at, or near, one of the intersections. Sometimes a horizon line is placed near one of the lines to emphasize either sky or ground. Most amateurs who have never read or thought about composition tend to put the dominant feature dead center in the viewfinder. In many cases, this is not as interesting as nudging the subject off center and the Rule of Thirds helps do that. 

However, if the subject and background is symmetrical, dead center can work well.