As mentioned in the comments, you are outside of the loop, so get_field will not know the ID. You can use the code below:
<?php
$meta_description = get_field('meta_description', get_queried_object_id());
if(empty($meta_description)) {
$meta_description = 'This could be an interesting meta description';
}
?>
<meta name="description" content="<?php echo $meta_description; ?> "/>
If you use var_dump() in the header.php file, you can just add it at the very top, followed by a die() to test:
<?php
var_dump(get_queried_object_id());
the_field('meta_description', get_queried_object_id());
die();
?><!doctype html>
<html <?php language_attributes(); ?>>.....
Give that a try and let us know how you get on.
Update: You may also want to look at something like the WordPress SEO plugin as it lets you manage your SEO meta tags without having to edit any code.