The rule of thirds is well known technique among the professional photographers. The rule of thirds is said to be used because if you place a point of interest on the lines it will become a more balanced photo. Studies have shown that when viewing images that have been taken using the rule of thirds, peoples eyes usually go to one of the intersection points.
The rule of thirds is when you imagine to break an image down to thirds just like what the rule sounds like. Both horizontally and vertically so you will have 9 rectangles in total. You don't have to imagine it like most professional photographers if your camera has a function that allows it to show on the LCD display.
An good example would be this picture of a duck on a lake. As you see bottom right intersection is over the ducks. This makes the duck have more attention placed on it.