。
She played a piece on the piano.
Literal
She [topic-は] piano [with-で] one-piece played.
曲 is the counter for musical pieces — songs, instrumentals, classical works. 一曲 reads いっきょく ('one piece') with the typical counter sound shifts: 二曲 (にきょく), 三曲 (さんきょく). The same kanji 曲 standalone can mean 'tune, melody, piece of music' as a noun — so 曲 functions both as the counter and the head noun, depending on context. で marks the piano as the means by which she played.