This is an email address that all 'undefined' email addressed to the domain will be sent to. For example, if there is no forwarder or account defined for enquiry@yourdomain.com.au, but somebody sends an email to that address, if you have set up a catch-all address it will automatically be forwarded to the email address defined as your catch-all address. This is especially useful for catching email messages in your system that may have otherwise been returned to the sender as address unknown. The downside to this is a lot of unwanted email will end up being collected by a catch-all address.
Please note that you are better off creating several email forwarders for different addresses you may want to "catch", rather than creating a catch-all address which will end up receiving a lot of SPAM.
