WordPress at a glance
function is not described

wp_admin_bar_add_secondary_groups() WP 3.3.0

Add secondary menus.

No Hooks.

Return

Nothing (null).

Usage

wp_admin_bar_add_secondary_groups( $wp_admin_bar );
$wp_admin_bar(WP_Admin_Bar) (required)

Code of wp admin bar add secondary groups: wp-includes/admin-bar.php WP 5.2.2

<?php
function wp_admin_bar_add_secondary_groups( $wp_admin_bar ) {
	$wp_admin_bar->add_group(
		array(
			'id'   => 'top-secondary',
			'meta' => array(
				'class' => 'ab-top-secondary',
			),
		)
	);

	$wp_admin_bar->add_group(
		array(
			'parent' => 'wp-logo',
			'id'     => 'wp-logo-external',
			'meta'   => array(
				'class' => 'ab-sub-secondary',
			),
		)
	);
}