A tax file number (TFN) is a unique personal identifier issued by the Australian Taxation Office (ATO) for tax and superannuation purposes. You apply online through the ATO after you arrive in Australia, and processing usually takes about 28 days.
Working holiday makers are taxed at a flat rate of 15 per cent on income up to A$45,000 earned from an employer registered as a working holiday maker employer. Having a TFN is what lets that concessional rate apply correctly.