A prerequisite course is a course that must be completed before a student can enroll in another course. It exists because the later course assumes the student already has certain knowledge or skills.
Why prerequisites exist
Prerequisites protect the learning sequence. For example, a student may need algebra before calculus, introductory biology before anatomy, or basic programming before a data structures class.
Prerequisites are not always classes
Sometimes the requirement is a test score, placement level, instructor approval, or minimum grade. A course description or syllabus may list what is required.
A prerequisite is basically the academic "start here first" rule for a subject path.