。
She practiced the piano over and over.
Literal
She [topic-は] many-times piano [object-を] play [of-の] practice [object-を] did.
練習をする ('practice') is the standard frame for talking about deliberate practice. The activity being practiced is built up through a verbal noun phrase: ピアノを弾くの ('the playing of the piano') is nominalized with の and paired up with の practice. The bracketing of two の particles in close range — ピアノを弾く + の + 練習 — creates a noun-modifying chain that's perfectly grammatical but rarely seen in single-line sentences. 何度も pushes the repetition into the sustained-effort range.