If you don’t renew your domain it will lapse. Your website will not load and you will not be able to receive emails through that domain.
If you have an Australian domain it will be locked for a number of days, and within this time you will be able to renew the domain. After this time, someone else will be able to register your domain.
Other domains including .com have quite different rules regarding expiry and if not renewed by the due date can often lead to loss of the domain.