<?php site_url( $path, $scheme ); ?>
Parameters
$path: (optional) Path to be appended to the site url.
$scheme: (optional) Context for the protocol for the url returned. Setting $scheme will override the default context. Allowed values are ‘http’, ‘https’, ‘login’, ‘login_post’, ‘admin’, or ‘relative’.
It simply overrides the default parameters in URL.
For example if you have a website URL like http://www.example.com (without https) and you want to add https URL on a specific link instead of http, then you use $scheme parameter.