A decline happens when the transaction fails. This can happen because of a number of reasons. See the table at the end of this page for some common decline codes and reasons.

You can find the decline codes in a transaction's webhook.

When a payment is declined by the payment processor, the user is sent back to your site where they are presented with a payment declined message. The user is then given the option to try again with a different payment processor. 

The URL for the payment declined page is yourdomain.com/payment-declined

Please see this article for a detailed list of all decline codes and explanations.