。
She has an ear for music.
Literal
She [topic-は] music [subject-が] understands.
わかる takes が, not を — a perennial stumbling block for English-speaking learners, since 'understand' feels transitive. In Japanese, the thing understood is the grammatical subject of 'understanding-ness happens.' 音楽がわかる carries a sense beyond literal comprehension: it implies sensitivity to music, the ability to appreciate it on its own terms — an ear for it, taste, refinement. Compare 冗談がわかる ('gets jokes / has a sense of humor').