An ABN is the 11-digit identifier for a business in Australia. It must appear on every tax invoice and, in practice, on every invoice a business sends to another business.
Having an ABN doesn't mean a business is registered for GST — that's a separate registration, required once GST turnover reaches $75,000 a year. If a supplier doesn't quote an ABN, a business customer may have to withhold tax at the top rate from the payment.