In music the word composition can be used in two ways.
A composition is a piece of music (the word "composition" means "putting together", so a composition is something where music notes have been put together). When a composer writes a piece of music he or she is making a musical composition.
The word "composition" can also mean learning the skill of how to compose. Music students may go to music college to study composition. They will do this by looking at famous pieces of music to see how the composers of the past wrote music. They will study musical form, harmony, orchestration, counterpoint and learn about all the instruments and how to write well for them so that it sounds nice.