A course is a collection or sequence of quizzes compiled by an author. The course markbook collates the percentage marks from all the quizzes in one course, and converts the average of these to a grade. Especially if the course has been taken over several weeks, this is a highly reliable indicator of current grade attainment.
The Markbook can be accessed from any assignment, but the version accessed via the More menu is more powerful, albeit harder to learn.
Use the filters to select multiple student sets (e.g. a whole year cohort), look at only specific attempts or change the sort order.
A note about filtering by attempts: the attempts filter includes the option to view only the best result from a number of attempts. We recommend you do not use it. For you, it will skew the recorded grading unrealistically higher. For the students, it tends to reward luck rather than diligence.