A "section" is a grouping of students. It might represent a physical classroom, or it might be a cohort of students within a classroom that you group together they have similar aptitude.
The reason we don't call them "classrooms" on the membership description (and instead use a confusing name like "sections") is because we want to help teachers understand that a membership cannot be shared among classroom teachers.
Three teachers cannot band together and share an account that supports 3 sections. But, one teacher could split her one classroom into 3 sections.