A prerequisite is a course which a student needs to take before taking a higher course. The prerequisite course gives the necessary background for the higher course. For instance, MATH 11 is the prerequisite of MATH 14.
If a student registers a course without taking its prerequisite, he will automatically be dropped from the course.
If the student took the prerequisite but failed it, he can still take the prerequisite and the higher course at the same time. He can apply for waiver of prerequisite if the instructor certifies that he have fully attended the prerequisite course.
On the other hand, the student can apply for a waiver of prerequisite even if he had not fully attended the course, only if he is graduating within a year from the time of his application for the waiver. Also, his request will be approved only if there is a high probability that he would pass both courses, based on his grades.
If the student is applying for a waiver of prerequisite for the course that he would take next semester, he should file his application on the last day of submission of final grades for the current semester.