discount coupon has applied on complete order not specific product in the order
In admin/commerce/discounts/manage/ choose discount type to product discount and add the product(SKU) for which you want to apply discount and save the discount form.

For adding description you can add it in form_alter()

/**
 * Implements hook_form_alter().
 */
function my_module_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'commerce_checkout_form_checkout' ) {
    $form['commerce_coupon']['#description'] = t('Enter the coupon code &lt;strong&gt;***100&lt;/strong&gt; to avail discount');    
  }
}


Hope it works.