Enabling this feature means that once your AUD balance, generated by sales via the Point of Sale, reaches the withdrawal threshold you set (minimum AU$100), the AUD will be automatically withdrawn to the bank account you specified, free of charge.
This setting can always be changed via 'Settings' → 'Point of Sale' → 'Auto withdraw AUD to bank account?'
Auto withdraw needs 'Auto convert bitcoin to AUD?' to be on. Turning Auto convert off also turns Auto withdraw off and clears the threshold. While Auto withdraw is on, the threshold is shown under the toggle and can be changed there at any time.