When you do poorly in a subject and get sent to a class that focuses on basic concepts and better study habits, you are taking a remedial class.
If you know that a remedy is a type of medicine, then you have a clue to the meaning of remedial. Like a remedy, remedial classes are supposed to improve you, specifically in school subjects that you haven't done well in. Lately, the term is most often used to describe courses given to students who are not prepared for regular work. New college students often have to take remedial classes before they can begin actual university-level courses.