How the elements inside the frame are exhibited to the viewer. Its about what to show, and what not to show, and how to show it or how not to show it. It enhances the meaning of what you want to say. What can we say with composition? There are some compositional trends that we see in most films and video content, such as rule of thirds, the golden ratio, and the triangular composition, these are pleasing to the eye. Few films go against this formula, this also helps the director tell a story to the viewer.
http://www.elementsofcinema.com/cinematography/composition.html
Sound and Visual 