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. The report can also show if the student has completed another course(s) that is marked as a "completion dependent" course.
Course completion does not have a "restrict access" function. It is best thought of as a report.