discount coupon has applied on complete order not specific product in the order commence ( commence discount ).coupon code is applicable  only one product.  but As per attached image coupon has redeemed price on  all product in the cart.i am explaining in details here.There are two product with name
1.Preventing Sexual Harassment at the Workplace - Certificate Programme with original price ₹3,596.00
2.Intellectual Property: Law and Management - Certificate Programme with original price ₹3,596.00

i have applied 20 % coupon only first product it should be redeemed -₹899.00 but it has redeemed -₹1,798.00 rather than ₹899.00 it means it is redeeming same amount on another product also however i have not applied coupon on 2nd product.

HELP WOULD BE GREAT APPRICATED
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.