예약이 · 예약을
reservation, booking
Example
인기 있는 식당이라서 일주일 전에 예약을 했어요.
It's a popular restaurant, so I made a reservation a week ahead.
Word parts
예 in advance+약 promise
Literally 'promise in advance'.
Good to know
For tickets (movies, trains), Koreans usually say 예매; for tables and rooms, 예약.