教室 (kyōshitsu) means "classroom." You use it to refer to a room in a school or other educational institution where lessons are taught. It is a specific place designed for learning activities and instruction. For example, 「教室で勉強します。」 (I study in the classroom.)

学校などで授業を行う部屋のことです。学習のための場所を指します。「教室で勉強します。」