Need to add a small portion of description to your WordPress nav_menu like above? Follow this simple step to do so.

Step 1 – add custom walker to your functions.php

if( !class_exists('Custom_Description_Walker') ):
////////////////////////////////////////////////////////////////////
// add description to wp_nav
///////////////////////////////////////////////////////////////////
class Custom_Description_Walker extends Walker_Nav_Menu {
    /**
     * Start the element output.
     *
     * @param  string $output Passed by reference. Used to append additional content.
     * @param  object $item   Menu item data object.
     * @param  int $depth     Depth of menu item. May be used for padding.
     * @param  array $args    Additional strings.
     * @return void
     */
function start_el(&$output, $item, $depth, $args) {
$classes = empty ( $item->classes ) ? array () : (array) $item->classes;
$class_names = join(' ', apply_filters('nav_menu_css_class',array_filter( $classes ), $item)
);

! empty ( $class_names )
and $class_names = ' class="'. esc_attr( $class_names ) . '"';
$output .= "<li id='menu-item-$item->ID' $class_names>";

$attributes  = '';

        ! empty( $item->attr_title )
            and $attributes .= ' title="'  . esc_attr( $item->attr_title ) .'"';
        ! empty( $item->target )
            and $attributes .= ' target="' . esc_attr( $item->target     ) .'"';
        ! empty( $item->xfn )
            and $attributes .= ' rel="'    . esc_attr( $item->xfn        ) .'"';
        ! empty( $item->url )
            and $attributes .= ' href="'   . esc_attr( $item->url        ) .'"';

// insert description for top level elements only
// you may change this
$description = ( ! empty ( $item->description ) and 0 == $depth )
? '<small class="nav_desc">' . esc_attr( $item->description ) . '</small>' : '';

$title = apply_filters( 'the_title', $item->title, $item->ID );
$item_output = $args->before
            . "<a $attributes>"
            . $args->link_before
            . $title
            . '<br /><span>' . $description . '</span>'
            . '</a> '
            . $args->link_after
            . $args->after;

// Since $output is called by reference we don't need to return anything.
$output .= apply_filters(
            'walker_nav_menu_start_el'
        ,   $item_output
        ,   $item
        ,   $depth
        ,   $args
        );
    }
}
endif;

Step 2 – add custom walker to your wp_nav_menu()

<?php wp_nav_menu( array( 'theme_location' => 'primary', 'container' => false, 'menu_class' => 'sf-menu', 'fallback_cb' => 'revert_wp_menu_page','walker' => new Custom_Description_Walker  )); ?>

add the ‘walker’ => new Custom_Description_Walker to your existed wp_nav_menu()

Step 3 – adding the description for each menu item

go to wp-admin -> appearance -> menu and click on the top right Screen Options and check (tick) the red description area

open any menu item and text into description box like below

Step 4 – add some css to the description

in this case, description were wrap inside a < span > so we need to add some extra css to span. example like below

.sf-menu a span { font-size: 10px !important; }

Once every step completed, you should see a small description in each of your menu item.