This shows how well you can express your flowing feelings and spontaneous creativity. In some dreams it is link with what your feel about sex or your genitals, and how well you express pleasure and feelings sexually. If it is a large and complex instrument it might be depicting the mind.
Musical instruments, or music in your dream, are a way you begin to become aware of subtle or creative impulses that are emerging. Just as a plant takes shape and colour from a tiny seed, so also your personal growth emerges from obscure and formless possibilities within you. Music is the expression of that flow to life.