Anurag wipes the Kajal off Prerna's face and says the stain is now gone. I presume this is meant to say that he will help wipe away any stain on Prerna's character as a result of her engagement to Naveen Babu.
I loved the other lines said to her as well: I should never have doubted you, I know you can't do anything wrong, it always necessary to always be so good etc etc. This may not be love yet but this guy really like this girl!