The index.php file is a template file. It defaults to being the homepage. Since our current version of wp-content/themes/customtheme/index.php is stripped down to only have an instance of The Loop, the Admin Tool Bar went missing on the front-end of the site. To add it back, we learn that a special Wordpress function called wp_footer() is what displays it.
This is a different function from get_footer(), which retrieves and injects the template partial called footer.php, just as get_header() retrieves the partial called header.php. We'll create our own partials shortly.