This is an instance of class WP_Rewrite, which is used to enable Pretty Permalinks in WordPress. It is set in the [wordpress root folder]/settings.php and the
WP_Rewrite is defined in [wordpress root folder]/wp-includes/classes.php.
The linked codex document has all the details about the class and the global variable’s usage. In a nutshell, the plugin authors can use this global variable to extend and customize rewrite rules and permalinks.
Back to full list of global variables.