The code is using the WordPress function `wp_enqueue_style` to add...

The code is using the WordPress function wp_enqueue_style to add a stylesheet called "plast-reset" to the current page. The stylesheet file is located at the URL returned by get_template_directory_uri() with the path "/assets/css/reset.css". The third argument of wp_enqueue_style is an empty string, which is used for the stylesheet's dependencies (i.e. other stylesheets that this stylesheet relies on). Lastly, the fourth argument $plast_version is a variable that holds the version number of the stylesheet, which can be used for cache-busting purposes.