The error pages are pages that will show up if an incorrect request by a web page viewer has been made.

If you create a 404 error page, then users will see this page if they try to go to a page which does not exist on your site. 

400 - Bad Request 
401 - Authorization Required 
403 - Forbidden 
404 - Page Not Found 
500 - Internal Server Error

For more detailed documents, please visit https://docs.cpanel.net/display/68Docs/Error+Pages