Apple's two-factor authentication requests a user to enter a verification code that is sent to the user's iOS device before they can use iCloud web applications. Find My iPhone is the only app that does not require a verification code in the event that a device gets lost and you can't access it.
This extra security feature comes weeks after numerous celebrities saw their iCloud accounts hacked exposing sensitive information such as private photos.
Apple has added a 'remember this browser' toggle so that you must only enter a verification code once. Two-factor authentication is slowly rolling out to iCloud accounts, so you may not see it just yet.
Apple first rolled out two-factor authentication in March of last year in the United States. It later launched the extra layer of security in Australia, Ireland, New Zealand and the U.K, Canada, France, Germany, Italy, Japan and Spain.