In art, "composition" is the way that you arrange and place different elements.
In painting and photography, "composition" means where different objects, shapes, and colors are in the picture. In music, "composition" means which instruments are used, the melody that they play, and how the song fits together. You can even use the same word to talk about a piece of writing, like a poem or essay.
The word "composition" is countable, so you can say:
That's a really nice composition.
We can describe a composition as "strong" or "weak". A "strong" composition is good and a "weak" composition is bad.