WordPress at a glance
function is not described

add_cssclass() WP 1.0

No Hooks.

Return

String.

Usage

add_cssclass( $add, $class );
$add(string) (required)
$class(string) (required)

Code of add cssclass: wp-admin/includes/menu.php WP 5.2.3

<?php
function add_cssclass( $add, $class ) {
	$class = empty( $class ) ? $add : $class .= ' ' . $add;
	return $class;
}