。
She's good at playing tennis.
Literal
She [topic-は] tennis [object-を] play [thing-の] [subject-が] skilled is.
The frame [verb plain]+のが上手 ('good at doing X') uses the nominalizer の to wrap the activity, then が to mark it as the subject of the stative predicate 上手 (jōzu, 'skilled'). Like 好き, 嫌い, 得意, 苦手 and other adjectives of preference and ability, 上手 takes its object with が rather than を. Compare the simpler テニスが上手 ('good at tennis,' applied to the activity as a noun) — the のが variant lets you apply the predicate to a verbal action where simply naming a noun won't do.