A catch-all email alias means that any email addressed to your domain name which does not have its own separate inbox will end up being sent to the catch-all account. For example if I had an inbox for Jack@mydomainname.com and Jill@mydomainnname.com and a catch-all pointing to the Postmaster@mydomainname.com then if an email came in addressed to Jack@mydomainname.com it would end up in jacks inbox. If an email came in addressed to firstname.lastname@example.org it would end up in the postmaster inbox since there is not a separate sales inbox. If you do not wish to capture email for accounts which you have not specifically setup an email inbox for you should delete the catch-all account.
Catch-all accounts were a common feature about a decade ago before email spam was the problem it is today. The problem with a catch-all account today is that it means receiving much higher levels of spam. Spammers frequently address mail to every possible name they can think of at a domain name in an attempt to get some delivered. With a catch-all address all emails would be delivered.
Creating a catch-all account (not recommended*)
To create this catch-all account make yourself an email alias from inside the webmail and point that alias to the existing email account where you want to receive all email that does not have its own separate account, ie in the example above this would be postmaster. This catch-all inbox will immediately start collecting mail for all usernames you have not expressly setup already separately.
*The recommended method today is to create aliases for each unique name that mail SHOULD be delivered for INSTEAD of creating a single catch-all alias.