"Decline" is a function for you to decline a new match.
When you received a new letter from a new friend but you do not want to continue the conversation with this user, you can use the "Decline" function and state the reason you decline the letter.
You can select a reason among the list or input it yourself. Your reason will be sent to the sender and then this user will not be able to send you new letter again in the future.
Also, the declined letter will not be counted in your Sent:Received Ratio.