"Middle school" is a school that students go to before high school, usually from about ages 10-11 to 13-14.
This school is also sometimes called "junior high school". Technically, a middle school is for students from grades 6 to 8, while a junior high is for grades 7 to 9. The first format is a lot more common in the U.S. However, English speakers mostly use both terms interchangeably.