If you use a third-party app that stores and synchronizes data with iCloud, Apple will soon require you to use specific passwords for those apps to access information in iCloud. This will allow your actual iCloud password to be masked while still granting access in a secure way to those apps to retrieve, store, and synchronize information with your iCloud.
This allows third-party apps that do not support two-factor authentication to communicate with iCloud. To use app-specific passwords, your iCloud must have two-factor authentication enabled.
If you use iCloud with any third party apps, such as Microsoft Outlook, Mozilla Thunderbird, or BusyCal, you can generate app-specific passwords that allow you to sign in securely, even if the app you're using doesn't support two-step verification. Using an app-specific password also ensures that your primary Apple ID password isn't collected or stored by any third party apps you might use.
The feature is coming starting October 1, 2014.