。
She can play the piano.
Literal
She [topic-は] piano [subject-が] can-play.
The potential-form construction with が marking what would normally be the direct object — when a verb is in the potential, Japanese typically swaps を for が around the underlying object. 弾ける is the potential of 弾く, formed by attaching ~える to the consonant-stem, the regular pattern for u-verbs (書く → 書ける, 読む → 読める, 話す → 話せる).