one's mind is...머릿속 means “inside one’s head” or “one’s mind,” and 이 marks it as the subject. It is commonly followed by a word describing a mental state, such as 혼란스럽다 (to be confused) or 비다 (to be empty).
KO: 시험이 끝나서 머릿속이 텅 비었어.
EN: The exam is over, and my mind is completely blank.