The active elements (with blue icons) demands the student to respond. If you do not check the item as REQUIRED, the student have the option of skipping the response and still complete the course. 

If the unit has the "User needs to answer all required assessments and view all pages in order to access the next unit" checked of, the student will have to respond to all the required items to be able to move on in the course.

Note: That an element is required does not mean that the student has to get the question right to pass the course - just that he/she responds.