Decline codes are the standardized responses an issuer sends back when it refuses an authorization. Each code names a reason: 05 do not honor, 51 insufficient funds, 54 expired card, 14 invalid card number. They split into soft declines, temporary conditions that may clear on a retry, and hard declines that should never be retried.
Why it matters
Retry discipline is now a compliance issue: Visa and Mastercard both fine merchants who hammer hard declines, and excessive retries poison the very issuer confidence that approvals depend on. The decline mix is also intelligence. A wave of 05 codes signals issuers distrust your traffic, a pile of 51s in a subscription file is a dunning problem, and bursts of declines on sequential card numbers point at card testing. Merchants who log and review codes usually find their next fix in the data.