Apple started rolling out 2-step verification for Apple ID accounts a while ago. It is now in the process of rolling out the feature to some users of iCloud.com web apps. That means users who have this feature enabled will need to enter a special code before accessing certain web apps such as Calendar and Mail.
As Apple Insider has reported, it is not clear when this feature will be available to everyone. The process does make your life less convenient but protects your sensitive data and apps. Find My iPhone is accessible even without secondary authentication.