The basic purpose behind the imaginary lines in the “rule of thirds” is to break down an image into three-columns (both horizontally and vertically) to get 9 equal parts and at the points where the vertical and horizontal lines cross each other are your focal points. You place your main subject on or near these points to draw attention and create a sense of size or space. It is used to balance your design or photograph within the frame. It is also used to place the main focal point a bit off to one side instead of placing it in the center. Rule of thirds helps you to compose interesting and balanced layouts, and draw attention to a particular area.
Further, the mentioned below image will clear your confusion: