XML-RPC.php is a file dealing with the Remote Procedure Call or RPC on your server, and it is crucial for a WordPress website to function properly. It’s a feature of WordPress that enables data to be transmitted, with HTTP acting as the transport mechanism and XML as the encoding mechanism. It allows the operating systems running in a different environment to make procedure calls over the Internet; here procedure calls mean that there exists a software in another corner of the Internet web and is calling a function of another software in some remote location.
For example, let’s say you wanted to post to your site from your mobile device since your computer was nowhere nearby. You could use the remote access feature enabled by xmlrpc.php to do just that.
There are functions in it that are helpful in writing XML-RPC clients and servers. Apart from that, there are several functions in it which you can use.