。
She's good at playing the piano.
Literal
She [topic-は] piano [object-を] play [nominalizer-の] [subject-が] skillful is.
ピアノをひく ('play the piano') is nominalized with の, turning the verb phrase into a noun-clause that then takes が as the subject of 上手 ('skillful'). It's worth noting that 上手 is a na-adjective and that ピアノ is in kana here (rather than the kanji 弾く); kana spellings of common verbs are very widespread in Japanese prose, especially when the writer wants a softer, more readable feel.