Pay As You Go (PAYG) withholding is the system under which employers deduct income tax from your wages before paying you and remit those amounts directly to the Australian Taxation Office. It means tax is collected gradually across the year rather than in one lump sum.
After the end of the financial year on 30 June, you lodge a tax return that reconciles the tax withheld against the tax actually owed. If too much was withheld, you receive a refund; if too little, you have a bill to settle.