입실하다 = take/occupy a room (입 = enter, occupy; 실 = room).
It is a formal word used mostly in hospitals, hotels where rooms are a commodity, or other places where they deal with a large group of people distributed to many rooms.
(병원에서) XX 환자(는) 입실했나요? Has Patient XX checked into his room?
(호텔에서) 오늘(의) 입실률은 72 퍼센트입니다. Today's occupancy rate is 72 percent.
입실 시간 = 체크인 시간 = check-in time, 퇴실 시간 = 체크아웃 시간 = check-out time.