Nipple shields were invented to protect sore nipples. They are little plastic covers that fit over the nipple and areola (the dark part of the breast around the nipple).
They actually don't work well for moms with sore nipples, but they may help baby if she's having trouble latching. To work properly, nipple shields need to be sized properly for both mom's nipple and baby's mouth. They also need to be put on so that they draw in a lot of breast. And baby's milk intake needs to be checked too, since the shields could affect how much she can draw in.
It's best to see an International Board Certified Lactation Consultant (IBCLC) if you think you need to try a nipple shield to make sure you're using it properly.