The catch-all email address is used in cases of an incorrect name or misspelling at your domain. For example, any email that is sent to someone who is not listed in the POP or forwarding sections will still be received, even if the name has a typo or is out of date (as long as the domain name on the email address was specified correctly). The catch-all address is the email that was listed when the account was setup by default.
You may setup your “catch-all” email address as a POP account, forwarder, group-forwarders, or autoresponder. Keep in mind that the email address you specify as your “catch-all” MUST exist as one of these types of settings.