XML-RPC was a feature offered in older versions of WordPress that let you remotely interact with WordPress remotely. It's since been replaced by the WordPress REST API more details of which you can find here:
https://developer.wordpress.org/rest-api/
By default xmlrpc.php is blocked on all Nimbus servers due to the security risks it posts and the fact that it can be subject to brute force attacks which could potentially take down your WordPress site.
If XML-RPC is needed to be used on your site you can unblock it via your sites dashboard by clicking the toggle button as below:
