。
She was practicing the piano.
Literal
She [topic-は] piano [object-を] was-practicing.
練習する ('to practice') is one of the most generally useful Sino-Japanese verbs in everyday life — covering sports, music, language, anything that gets better with repetition. The past progressive 練習していた tells us this was an action in progress at some past moment, not a one-off practice. Piano lessons (ピアノのレッスン or ピアノ教室) are a near-ubiquitous extracurricular for Japanese children, especially girls, dating from the postwar middle-class boom in private music education.