that someone is a person사람이다 combined with 라는. It is commonly followed by 것 or 걸 to mean “the fact that someone is a person,” and can also modify a noun in the sense of “called” or “known as a person.”
KO: 그는 믿을 수 있는 사람이라는 걸 모두가 알고 있어.
EN: Everyone knows that he is a trustworthy person.