Most people tend to place their subjects, people or objects, in the center of the frame. But when things are centered...they seem almost too balanced and fail to catch our eye. The image is static.
Instead, our eye is drawn to things off centered. When something is slightly off... we notice it.
To make your photos more "interesting"...think of this rule of thirds as you compose your image.
For example....a landscape. Avoid having the horizon dead center, splitting the photograph in a top and a bottom. Instead, experiment with more sky, or more ground...just not in the middle. Use the 9 squares as a guideline. If there is an old barn, or a person, in the landscape...use the same rule. Place it/them off center, by about a third.