Elective units are "free choice" units that some courses have built into their structure. Elective units allow you to study a unit from any school or department to earn credit towards your degree. In order to take a unit as an elective, you still need to meet the pre-requisite requirements and the conditions of the unit. Some units have pre-requisites that restrict their undertaking to specific courses and cannot be studied outside of that course.
If you have 100 credit points of electives you may be interested in one of Curtin's minors.