May 16th 2017
By Mike Flaminio
Beginning on 15 June, app-specific passwords will be required to access your iCloud data using third-party apps such as Microsoft Outlook, Mozilla Thunderbird, or other mail, contacts and calendar services not provided by Apple.
If you are already signed in to a third-party app using your primary Apple ID password, you will be signed out automatically when this change takes effect. You will need to generate an app-specific password and sign in again.
This is kind of a pain in the butt, but good account management. The app-specific password sets up authenticated access to connected devices using a unique password. If one system is compromised, the rest of your access, including your iCloud account itself, should be secure. Furthermore, it makes it easy to control access by revoking access to apps.
So it's a pain to setup, and a pain again when you change your password, but it helps keeps your accounts locked down.