Why do american schools have periods?

Most Americans ive seen talk about school always say "I have 1st period with friend A and second period with friend B" and im just like why??? In Serbia from 1st to 4th grade we go to every class with the same people and in 5th to 8th we get put in new classes but even then we all have the same classes. And is it true that some periods last an hour? In my school a class is like 45 minutes 😭