A "catch-all" e-mail address is simply an e-mail address that will receive e-mails that have been sent to a non-existent account at(@) your domain. This means that if John@Smith.co.uk and Bob@Smith.co.uk are the only e-mail accounts created and someone mistakenly tries to send a mail to Robert@Smith.co.uk, this e-mail would be delivered to the catch-all address if one is set up. If no catch-all e-mail address is set up, then the e-mail would simply be returned to the sender with a message that it is not deliverable because no such e-mail account exists.
With a catch-all e-mail account set up, John@Smith.co.uk and Bob@Smith.co.uk will continue to only receive e-mails that are addressed to them and will not receive any mail addressed to non-existent accounts like the catch-all e-mail.
A catch-all e-mail address can be made by using an asterisk(*) as the account name such as *@Smith.com.