It could be anything, but if you’re offering a service of some kind, rather than a simple downloadable product, you may need to trigger something to happen when an order is complete. Here’s a simple snippet illustrating how you can add an action to woocommerce_payment_complete. Add something like the following to your functions.php. You’ll need to grab the product_id from the admin side. I’ve also added in how you can retrieve the user’s information at the same time in case you need something from there as a part of your trigger. Enjoy!

add_action('woocommerce_payment_complete', 'custom_process_order', 10, 1);
function custom_process_order($order_id) {
    $order = new WC_Order( $order_id );
    $myuser_id = (int)$order->user_id;
    $user_info = get_userdata($myuser_id);
    $items = $order->get_items();
    foreach ($items as $item) {
        if ($item['product_id']==24) {
          // Do something clever
        }
    }
    return $order_id;
}