Simple Definition:

Composition is the arrangement or combination of different elements [parts] to create a unified whole. In art and music, composition refers to the organization and structure of visual or auditory elements [things you can see or hear] to create an overall composition. For example, in a painting, the arrangement of colors, shapes, and lines can be considered the composition.

Very Simple Definition:

Composition is how things are organized to look or sound pleasing. For instance, when you arrange flowers in a vase to make them look pretty, that’s composition.