Query WordPress posts by a custom field value

$loop = new WP_Query(array(
    'post_type'  => 'product',
    'meta_key'   => 'in_stock',
    'meta_value' => 'yes',
));

while ($loop->have_posts()) {
    $loop->the_post();
    the_title('<h3>', '</h3>');
}
wp_reset_postdata();