When someone follows a link to a page that does not exist yet, DokuWiki will send a usual 200 HTTP response. In a wiki, this is a desired behaviour, however if DokuWiki is used as a CMS system, one might prefer to have DokuWiki answer with a 404 “not found” response. Setting send404 to 1 will cause this behaviour.
Note: this will not change what content will be sent, only the HTTP status changes.