This song is a beautiful love letter where Gran Rah compares his beloved to music itself! He sees her as the missing piece in the world, an "inspiración de pintores y poetas" [inspiration for painters and poets].
He uses so many musical metaphors to describe her. Her body is a guitar, her hair is the strings of a harp, and her chest is a drum. He even says her voice harmonizes with his, and her beauty makes him think, "eso sí que es músico" [that, yes, is music]. It's a really creative and passionate way to express how deeply he feels about her, seeing her as the ultimate melody in his life.