Units are not like units in the HSC. When she says units, she means subjects.
Units can be compared to credit points at university. You need to complete a certain amount of units at high school, and you need to complete a certain amount of credit points at university. The difference is, sometimes some subjects are worth more credit points than normally expected, which means there's more work involved (in theory), or the subject lasts longer than a semester (the whole year for example), others have less credit points which could mean there's less work involved over the semester, or the length of the subject is smaller.
For many degrees you get given a list of subjects you have to complete. However, there is also an amount of credit points which must be completed. Much of the time, the list of subjects makes up less credit points than required, which means you have to do elective subjects to make up the rest.
For example: BCompSc at Imaginary University which has 10 credit point subjects (some universities have three, some have six, some have eight, some have ten and some even have fifteen), the degree length is three years. A standard full time study load would be 40 credit points (four subjects), a semester. So in a year, 80 credit points (eight subjects) would normally be completed. A student must reach at least 240 credit points in three years or more to graduate.
Core subjects (subjects which must be done):
MATH100 - Mathematics I - 10 cp
MATH101 - Mathematics II - 10 cp
MATH111 - Discrete Mathematics - 10 cp
MATH112 - Probability - 10cp
COMP101 - Introduction to Information Technology - 10 cp
COMP150 - Introduction to Programming - 10cp
MATH200 - Mathematics for Computer Scientists - 8 cp
COMP200 - Advanced Programming - 10 cp
COMP210 - Database Management Systems - 10 cp
COMP220 - Network and Distributed Computing - 10 cp
COMP230 - Ethics in an Information Technology Environment - 10 cp
COMP300 - Computing Project - 20 cp
COMP301 - Operating Systems Design - 10cp
Total credit points taken up by core subjects: 128 credit points
Total credit points to be taken as elective subjects: 112 credit points or more.
These electives can be taken from anywhere the course handbook allows. Some degrees have strict rules regarding electives, and provide a list of subjects from which you can choose from. Others might be more relaxed and say you can choose electives from anywhere (so long as you meet the requirements of that subject).
You'll pick it up very easily. Don't worry.