I suggest you have a read of the UAC site.
http://www.uac.edu.au/apply/offer_rounds.html
Basically, the main round is the round where the majority of students get their preferences.
When you submitted your UAC application you make a list of 9 courses that you want to do, with the 1st course being the course you want to do the most, and going down the list to your least preferred course.
When the cut-offs are released, you can check the course that you applied for and check whether you met the cut-off for it, or alternatively log into the UAC website to see what course you were offered.
The late round follows the same concept as the main round, except it is another round held after the main round for those who wish to change their preferences and hope to get into a different course.