What is a catch-all mail forward?
When someone sends you an e-mail but mistypes the address, you can still receive that e-mail by using the catch-all function.
Someone sends an e-mail to firstname.lastname@example.org instead of email@example.com. Typically this e-mail will be rejected but with the catch-all function you are still able to receive this e-mail.
Caution: Lots of spammers use this function to send e-mails to your domain by using randomly generated e-mail addresses. We therefore strongly advise you not to enable this option unless absolutely necessary.