A catchall mailbox is a mailbox to which any email not destined for a specific mailbox will be delivered. This is better explained with an example:
In this example, any email sent to john@dom.com will be delivered to the corresponding mailbox, as will jan@dom.com be delivered accordingly. However, what happens if an email is sent to zxy@dom.com? As this address has no specific mailbox assigned, normally the email would be rejected with the message “user unknown”. However, because of the catchall entry, rather than being bounced the email will be delivered to the catchall mailbox (catchall@dom.com). Similarly, any other addresses which do not have specific mailboxes assigned will be caught by the catchall mailbox and delivered to this address. (if catch all in enabled)