Here what you should know about Course Completion:
Course completion shows if a course has been completed. It can show the progress a student is making towards finishing the course according to specific criteria. The criteria can include meeting an activity's grade level or a manual checking "complete" by either the student and/or teacher.
Before setting up your course completion criterion please make sure that Completion Tracking is enabled site wide under Site Administration > Advanced Features > Enable Completion Tracking:
After Completion Tracking is enabled site wide, go to your Course(s) > Settings > Edit Course Completion Settings. Select the criteria that needs to met in order to complete the course.
Here you can find more detailed information about Course Completion, its settings and usage.