。
She's just about to start playing the piano.
Literal
She [topic-は] piano [object-を] play-begin [about to-ところだ].
弾き始める is a compound verb pairing 弾く ('to play, pluck') with 始める ('to begin'). The pattern verb-stem + 始める is highly productive in Japanese: 食べ始める ('start eating'), 走り始める ('start running'), 話し始める ('start speaking'). ~ところだ marks an action at a particular point on its timeline — here, the just-about-to-do moment. Combined with 弾き始める, the whole phrase says 'is at the point of starting to play,' a finely sliced moment of imminent action.