。
She's good at playing the guitar.
Literal
She [topic-は] guitar [object-を] play [thing-of-のが] is-good-at.
A textbook のが nominalization: a clausal action (ギターを弾く, 'play the guitar') gets nominalized with の, marked as the subject with が, and predicated by a stative ability/preference adjective like 上手 ('skilled at'). The same template fits any verb-of-activity slotted into the same frame. The verb 弾く ('play [a stringed/keyboard instrument]') takes ギターを — same general object marking as for striking instruments (ドラムを叩く) and wind instruments (フルートを吹く), but the verb itself differs by instrument family.