Doctrine of mortgagee in good faith

Dear PAO,

My sister forged the signature of my deceased parents to make it appear that a certain land was donated to her. She was able to register the lot in her name. I also found out that the property was mortgaged to a certain businessman because of the annotation at the back of the title. I informed the businessman about the forgery but he claimed that he is a mortgagee in good faith. Please enlighten me what he meant by being a mortgagee in good faith.