This is one of the best known principles or techniques for composition in photography and acts as a guideline for how an image should be composed for the most aesthetically pleasing result.
A Little More Detail:
With the rule of thirds, the frame is divided into nine equal parts by mentally drawing two lines vertically and two lines horizontally across it. The rule dictates that the focal points or important parts within the scene of your image should be placed in line with these mentally-drawn lines or at the intersections of the grid.