WordPress - Add description to featured image field

custom.php

/*
*  Add description to Featured image box
*/

function new_post_thumbnail_meta_box() {
    global $post;
     
    $thumbnail_id = get_post_meta( $post->ID, '_thumbnail_id', true ); // grabing the thumbnail id of the post
    echo _wp_post_thumbnail_html( $thumbnail_id ); // echoing the html markup for the thumbnail

    echo 'The field description goes here';
     
}
function render_new_post_thumbnail_meta_box() {
    global $post_type; // lets call the post type
     
    // remove the old meta box
    remove_meta_box( 'postimagediv','post','side' );
             
    // adding the new meta box.
    add_meta_box('postimagediv', __('Featured Image'), 'new_post_thumbnail_meta_box', $post_type, 'side', 'low');
}
add_action('do_meta_boxes', 'render_new_post_thumbnail_meta_box');

thanks for your share

Thanks for sharing.

Instead of writing __('Featured Image') you should write _x('Featured Image', 'post').

works for me, thanks for sharing