After recent celebrity hacks, Apple have been forced to make their security a little tighter. If you use third-party apps like Outlook that access Apple’s iCloud, you now have to have a two-step authentication process. Which means your device will now ask for a separate “app-password” for each app that accesses you iCloud. Apple have already updated iCloud with an extra security layer used to protect accounts by sending a four-digit code to your personal device. However, many third-party calendar, contact and email apps that access iCloud don’t support a two-step authentication, and could therefore expose your iCloud password, giving hackers the opportunity to collect your personal data.
Apple says if you are signed into those apps when the change goes through today, you will be automatically signed out and prompted to generate a new password
To see how click here for more details