Major courses are courses that are selected to be related to your specific department. So if you score an "F" on those courses, there is a chance of lagging if they become a prerequisite for other courses next semester. Example: If course "Course K" is "major course" and it is a prerequisite for another course called "Course L" you can't take "Course L". This may continue to final year and cause lagging. Hope it is clear, man 🤗