I am creating a custom blog system like wordpress.

However , I did not know very well what the <link rel="pingback" href="http://localhost/wordpress/xmlrpc.php" /> is perfect for, and whether it's essential for better page ranking.

If it's necessary, can you really implement it during my custom website? how?

Thanks.

XML-RPC is a method to send data (as xml files) to wordpress. For instance you can remotely publish posts utilizing an offline blog editor.

Personally I do not believe it is really helpful and it is certainly not essential.