Each of the digits from 1 to 12 signifies a corresponding letter grade. (A+ is 12, A is 11, etc.).
Refer to
http://registrar.mcmaster.ca /calen...ent/pg145.html
The CA is your cumulative average. So if you average out all your courses, it should be a minimum of 7. This doesn't necessarily mean you have to get a 7 in all your courses, but you do have to keep up the average.
If you fail a course then that could have some detriment to your CA seeing as it will bring it down. You having to retake a course would depend on whether or not the program you want to get into requires satisfactory completion of that course.