App-specific passwords are passwords for your Apple ID that let you sign in to your account and securely access the information you store in iCloud from a third-party app. For example, use app-specific passwords with mail, contacts, and calendar services not provided by Apple. App-specific passwords maintain a high level of security and help ensure your Apple ID password won’t be collected or stored by any third-party apps you use.

If you are signed in to iCloud for Windows, you don't need an app-specific password when using your Apple ID with third-party Microsoft apps.

To generate and use app-specific passwords, your Apple ID must be protected with two-factor authentication.

You also need an app-specific password for your devices that use Mac OS X Lion 10.7.5 and earlier or iOS 5 and earlier. If you don’t have devices that can be updated to iOS 9 or later or OS X El Capitan or later, you can set up two-step verification and generate app-specific passwords.