A reason code in relation with chargeback is an official explanation that is issued by the card-issuing bank, which tells the merchant why a chargeback was filed. Do not consider it just like a label. It basically dictates the entire strategy of how you will fight the dispute that has been raised by your customer.
Every chargeback comes with a specific code which is tied to the category, like fraud, authorization issue, or service-related disputes. You should think of it as the battlefield intel. If the reason code says fraud, then your evidence needs to prove the cardholder actually authorized the transaction.
If it says product was not received, then you must ensure that you provide delivery confirmation.
Misreading the reason code is where most merchants lose the cases. They send the wrong evidence and it immediately weakens their case.
Each card network and bank has its own set of reason codes, which makes things a little bit more complex. What looks like the same issue can actually have different reason codes depending on the card network. That is why it is important for serious merchants like you to not treat disputes in a casual way. You should build structured workflows around the reason codes and then provide substantial evidences to win the disputes. The bottom line here is simple. The reason codes are not admin details. They are basically your playbook for winning or losing money. You should master them and this will help you shift from reacting to chargebacks to controlling them.