From the photo I can't tell for sure whether or not that bar is connected to the suspension. If it's connected to the suspension on each side then it's an anti sway bar, otherwise it's just a strut brace (body stiffener).
I notice that the car in the picture has the old hourglass mid-body brace (the short brace in the center of the chassis). If this picture is of your own Prius, I strongly recommend that you buy either the new tube-like Toyota brace or Rude person's's E-bay aftermarket brace. You'll be very pleasantly surprised either way - and it's a 5-minute DIY job.
I have a 2012 Prius Plugin. This was the only picture I could find of a Prius undercarriage. It was pushed up a bit to meeting the pan above it. Hope it isn't anything critical. It seems to drive fine
