The phrase 책을 펴세요 means “Open the book.” It is often used in classrooms when instructing students to start reading or referring to a specific section.
Examples:
Teacher directing students in class:
선생님: 책을 펴세요! 45쪽을 보세요.
Teacher: Open your book! Look at page 45.
Giving instructions in a study session:
책을 펴고 첫 번째 문장을 읽어 보세요.
Open the book and read the first sentence.
At the beginning of a lesson:
오늘 수업을 시작합니다. 책을 펴세요!
We are starting today’s lesson. Open your books!