ACN = Australian Company Number.
Upon company registration ASIC generates a unique nine-digit ACN as a primary identifier for the new entity. Although you can change your company name, you cannot alter this number. It is only issued to new companies and allows ASIC to monitor and administer your company.
If you decide to incorporate you will need to register your company and obtain an ACN. Can't decide on a company name? Your ACN can be your company name if you so desire.