Census date is the deadline for finalising your enrolment and fees for each teaching period. When this date has passed, you are committed to the subjects you're enrolled in. This means you will be charged tuition fees and receive a grade for all your enrolled units.
You may change your enrolment up to the census date (ie. withdraw from subject or change to another subject), but no changes can be made after this date. Thus students who withdraw from a subject after the census date will receive a Fail grade on their transcript and no refund will be paid.