Catchall is a nickname for a type of POP3 Mailbox or Forwarder.
A Catchall Mailbox will receive (catch)any email sent to the domain that does not already have a specific Mailbox or Forwarder set-up.
A Catchall Forwarder will forward any email sent to the domain that does not already have its own Forwarder or Mailbox set-up to the destination email address specified.
a Catchall cannot be allocated to a Mailbox and a Forwarder at the same time it has to be one or the other.
Catchalls can be used as a method of not loosing emails sent to a domain that have the user/alias part misspelt e.g. an email intended for joebloggs@domain.com accidentally sent to koebloggs@domain.com will end up in the Catchall Mailbox.
Another common use is to allocate the Catchall to a Mailbox purely to receive any email that is not destined for an existing Mailbox e.g. a company of 4 (Mark, Nick, Sam and Dave) set up Mailboxes for each individual and also set up a Catchall Mailbox so that the addresses sales@domain.com and info@domain.com etc. can also be used.