Apple is currently testing a way to make its iCloud system more secure by using two-step verification. Before the new system went into testing, all people needed was a simple password to gain access to iCloud.com. Using only a password makes access easier to gain by nefarious hackers.
iCloud now uses Apple's normal two-factor authentication system with users logging in using a password and a four-digit verification code. That four-digit verification code has to be sent to a trusted device.
Once that four-digit code is entered, iCloud apps are unlocked and can be accessed normally. The only iCloud app that can be accessed without the verification code is the Find my iPhone button. Find My iPhone is not secured with two-factor authentication to allow you to find your device if it is lost or stolen and is your trusted device.