Changes are coming soon to iCloud. App-specific passwords will be required to access users’ iCloud data. This change will take place on June 15th, 2017. 

Apple notified their customers this evening via email about the upcoming changes. Apps such as Microsoft Outlook, Mozilla Thunderbird, and many other email and calendar apps will be impacted by this.

When the change take place, users that are already signed in with their Apple ID will be automatically signed out.  To get back into the app, the user will have to come up with a new app-specific password to sign in again.

In order to do this, Apple recommends users generate an app-specific password by turning on two-factor authentication with your Apple ID and then follow the instructions detailed below.

To generate an app-specific password, turn on two-factor authentication for your Apple ID and then follow the instructions below:
Sign in to your Apple ID account page (https://appleid.apple.com)
Go to App-Specific Passwords under Security
Click Generate Password