It is associated to your iCloud keychain. When you setup iCloud keychain your existing devices can be used to allow new devices to use your keychain. If you no longer have access to your other devices and you dont know your iCloud keychain security code you will have to reset it due to you being unable to meet the security requirements (it helps to protect your passwords) Of course if you dont care about your iCloud keychain passwords or dont use them and are happy to reset and start again. You will still have to reset to continue.