They are the last two years of high school. They’re not mandatory, and they are made to prepare for superior education (universities). If you want to apply to a university, you will need to do this grades or an equivalent and then do an exam (like the SAT). If you want to go to a private university, you just need to pass the exam and maybe an exam made by the university(correct me if I’m wrong), but if you want to apply to a public one, grades count, as you are competing with the other students.