Relay means: Receive mails for that domain. But then send them somewhere else.
“Relay this domain” will receive any mails for that domain and send them further on to either the mailserver it finds in the MX record or use a defined sender dependent transport. Regardless if there is a mailcow user with a defined mail address.
“Relay all recipients” will only relay existing mail addresses defined in mailcow.
“non-existing Mailboxes” is the opposite, only non-existing mail addresses of that domain will be relayed.