Composition in photography is where a person or object in placed one third to the side so the viewer in drawn to that area. The most basic rule of Composition is to gain the viewers attention. This is also known as rule of thirds. The area of which the lines cross over on the grid is called the 'sweet spot' this is where the viewer is most likely to look.
Best and worst image (above):
The best image is the one on the left as it follows rule of thirds and it has the clearest/cleaner background so it makes you drawn to the tree and its shadow. The worst image in in the middle as it is overexposed, so that it makes us focus on the bright light/sun in the corner and not the tree.