Apple rolled out two-step verification for Apple IDs about a year-and-a-half ago. Now it's finally bringing the same added security measure (which sends a numeric code to your phone or other trusted Apple device that you need to enter after your password) to its iCloud website and apps. As observed by AppleInsider, users who already have two-step verification enabled on their Apple IDs may see their iCloud web apps — Contacts, Mail, Calendar, Notes, etc. — grayed out and "locked" until they enter in the code sent to their devices.
The only app that appears to be accessible on iCloud without two-factor authentication is "Find My iPhone," presumably because that one can't be used to access a user's personal data. AppleInsider notes that in tests, two-step verification only appeared for some iCloud accounts and didn't always prompt when logging off and back on, suggesting it's rolling out slowly. Still, it's a welcome addition to Apple's security measures given that the company has had recent trouble keeping its services and devices protected.