Pre-requisites are units you must have studied in order to be considered for admission to certain courses. They are put in place to ensure that when you enrol in a unit you have the background knowledge required to succeed in the unit. If you have not met the pre-prequisite you will not be able to enrol in that unit.
If you believe that you have sufficient knowledge to complete the unit (e.g. from previous studies at another university), you may apply for a Waiver of Unit Rules through self-enrolment on your MyUC account.