Adding short description to menu items

Copy the below code and paste it anywhere in the functions.php file:

class My_Walker extends Walker_Nav_Menu
{
	function start_el(&$output, $item, $depth, $args) {
		global $wp_query;
		$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';
	
		$class_names = $value = '';
	
		$classes = empty( $item->classes ) ? array() : (array) $item->classes;
	
		$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
		$class_names = ' class="' . esc_attr( $class_names ) . '"';
	
		$output .= $indent . '
';
	
		$attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
		$attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
		$attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
		$attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';
	
		$item_output = $args->before;
		$item_output .= '';
		$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;

        //Change the class in the span tag to the class you have in your menu item description span tag.
		$item_output .= '' . $item->description . '';
		$item_output .= '';
		$item_output .= $args->after;

		$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
	}
}

Copy the below code and paste it anywhere in the header.php file where you want to display your menu:

 'main-menu',
		'menu'            => '',
		'container'       => false,
		'container_class' => '',
		'container_id'    => '',
		'menu_class'      => 'none',
		'menu_id'         => 'none',
		'echo'            => true,
		'fallback_cb'     => 'wp_page_menu',
		'before'          => '',
		'after'           => '',
		'link_before'     => '',
		'link_after'      => '',
		'items_wrap'      => '
%3$s
',
		'depth'           => 1,
		'walker'          => $walker //Walker
		));
?>