When a transaction is declined an error code and error message (sent from your Authorize.net payment gateway) is presented to the online donor so they can correct their mistake. Sometimes the donor will want your help understanding the error. Getting more information on the error code can help with that effort.

Common error codes:

https://support.authorize.net/s/article/What-Is-the-Decline-Reason-Code-for-a-Transaction 

Look up any error code:

https://developer.authorize.net/api/reference/responseCodes.html