Hi hcha, did your turtle have that when you received it? That damage could have come from a bacterial infection or something like shell rot. My main concern would be that the area is healing and that the infection is over.
Since the damage looks like it was severe, there could still be an underlining infection underneath or in the surrounding area. At the very least, that area is susceptible to infection if not closely monitored.
It's usually a good idea to have a herp vet monitor the condition and determine if there should be any treatment options. I agree with megcornell that it looks like is scarring, but you do want to make sure that the infection is gone.

