If your payment gateway is not able to process a transaction, a response code indicating the reason as to why may display. In some cases, it may just list the code number while other times, it will give a full description as to why it is not able to take the payment. If a reason code is given, you may need to look it up depending on your payment gateway the response code as to why the payment did not process. Here's a link to some of the payment gateways that publicly list this information. Just to note, some payment gateways share the same response codes so you may be able to reference another site if your particular one is not listed:
Authorize.Net - https://developer.authorize.net/api/reference/responseCodes.html
BluePay - https://www.bluepay.com/downloads/BluePayGuide.pdf information found on page 26.