？
What are you doing?
Literal
What [omitted object-を] doing [explanatory-question-の]?
してる is the casual contraction of している (the ~ている progressive form). の at the end of a question is the casual explanatory/inquisitive ending — softer than just appending か, with a curious or probing tone. Asking 'what are you up to' is one of the most common conversational openers in Japanese.