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 "Year 13" website has a lot of information on Gap Year options and other programs. It's well worth a look here.