ABN is the abbreviation for Australian Business Number, it is a unique 11-digit number issued by the Australian Taxation Office (ATO) to each business in Australia who are eligible to have an ABN or for whom it is compulsory to have an ABN. ABN helps to differentiate your business from the other businesses in Australia.
If you are running your business in Australia, then it is more likely that either you would have applied for the ABN or you will apply for it in the future. ABN is just not a licence or certification; it is much more than that, it authenticates and validates your business for schemes and benefits by the Australian Government.