In very simple terms:
Major = topic you focus on in your course
Minor = topic you studied a bit but not as much
Each major and minor will have specific subjects/units combinations you need to complete to get that major or minor. In most courses you also have enough room in your coursemap to complete some units that don't contribute towards a major or minor for you. Each individual subject/unit contributes to your GPA and WAM.
I'm not going to comment on medicine specifically but I hope this helps regardless
