Plastic "Mud Flap(?)" in Front of Rear Tire?!?

[MadMax]

Was coming home from school the other night, hit something in the road and suddenly heard a bad noise from driver's side rear tire. Pulled over and found that this small plastic piece at the forward edge of the rear wheel well had broke and was rubbing up against the tire.

My questions are, what does this piece actually do, and how much is a replacement (if it needs to be replaced)?

Here's a pic of the piece I am talking about, in the red oval...



I took pics of mine, half of it is still on the car and the other half is actually in the car (i kept it in case I could re-attach it somehow). Unfortunately, due to the extreme cold weather we had here in southcentral Texas last night, we lost power for over seven hours and I was unable to upload them to my computer. I will do so and post them later.

Cheers! M2