Diana is the goddess of the hunt. Associated with wild animals and woodlands, she has the power to talk to and control animals.
Diana is reaching for her missing attributes: her left hand for the deer often accompanying her, and her right hand for the missing arrows on her back. You can still see the strap running diagonally across her breast which was intended for the quiver. Her dress, a short chiton, is waving as if she is in rapid movement, caught in a strong wind.
Gustav III got this statue as a bonus when negotiating for Apollo and the Muses, which you can see standing in front of the opposite wall in Gustav III's Museum of Antiquities at the Royal Palace.