It refers to the mailbox on a domain which catches all the emails addressed to the domain that do not exist in the mail server. This functionality gives you the ability to redirect void, invalid and improper emails sent to your domain to a “catch-all email address”. Mostly such emails are messages sent to your domain randomly to email accounts which do not exist. The default email address (catch-all), will “catch” any mail that is sent to an invalid email address for your domain.