Composition is the foundation of a work of art, in visual art, music, dance, theater, literature or film. In a broad sense it is the division into place, time and movement.
In visual art it is the layout of a specific surface or space. For example, a painting often involves the layout of a rectangular surface.
The composition determines both harmony and tension in the artwork. But it also determines the type of drama sought and the mood. With that also the expression.