The rule of thirds is a compositional rule of thumb in visual arts such as painting, photography and design.
The rule states that an image should be imagined as divided into nine equal parts by two equally-spaced horizontal lines and two equally-spaced vertical lines
Important compositional elements should be placed along these lines or their intersections
In other words: Before you take a picture, imagine your picture area divided into thirds both horizontally and vertically. The intersections of these imaginary lines (the pink circles) suggest four options for placing the centre of interest (or power point) for good composition. The option you select depends upon the subject and how you would like that subject to be presented.
The upper-right position has been picked for this subject so that we could see the full shadow and most of the tracks that lead to the seagull