？
What's she up to?
Literal
She [topic-は] what [object-を] is-doing [casual-question-の]?
Sentence-final の (after a plain form) is the casual explanatory question — direct, friendly, often used with intimates or to children. 何やっているの? ('what are you doing?') has a curious or slightly probing flavor, especially compared to the more matter-of-fact 何している? without の. やる is the casual cousin of する — both mean 'do,' but やる has a touch more grit and shows up in everyday speech with verbs of action. Sometimes やっている contracts to やってる in casual speech.