In our modern world of automatic cameras,which focus for us and adjust the exposure in an ever more perfect way (most of the time),the biggest difference between a good photograph and a mediocre one is the compositon.
In every photograph we take,we can decide where the boundaries of that photo will be,called the cropping.We can also choose the viewpoint.If we are taking pictures of people or movable objects then,often,we also have the opportunity to arrange them into the shapes we want.