。
She showed extraordinary talent at the piano.
Literal
She [topic-は] piano [in-に] extraordinary skill [object-を] showed.
腕 ('arm') stands in metaphorically for skill — the body part you use becomes the figure of competence. The phrase 腕を示す ('show one's skill') joins a family of 腕-idioms: 腕がいい ('skilled'), 腕を磨く ('polish one's skills'), 腕がなる ('itch to display one's skill'). 非凡 ('extraordinary') is the negation of 凡 ('ordinary, mediocre'), and as a na-adjective takes な before nouns. The に here marks the domain or area of the skill — 'at piano,' similar to 数学に強い ('strong in math').