The "Catch-All" account is the email account where all un-routed emails sent to your domain will be send to. So, if someone sends an email to 123@domain.com and you do not have an account "123", the email will be routed to your "Catch-All" account.
The "Catch-All" account is the email account where all un-routed emails sent to your domain will be send to. So, if someone sends an email to 123@domain.com and you do not have an account "123", the email will be routed to your "Catch-All" account.