A "Gap Year" is where you take a "year off" after your secondary schooling to do something else before you undertake further education and training (or full time employment).
Normally, people travel, work part time or do volunteer work, or a combination of these.
It is now a common practice to defer university for a year and undertake these activities before commencing university the following year. Students often use this time to work to gain "independent status" before going to university, however, I suggest you visit the Centrelink website to check on the current guidelines for this.
The buttons below will take you to information on different Gap Year programs that are available and also, a very common Gap Year activity - Volunteering.