Apple today activated two-step verification for the iCloud and iMessage services. This means that when a user tries to enter iCloud or iMessage from a new device, they will have to enter a number that is sent at that moment to the iPhone or another iOS device that you have registered. Obviously, if you are not the user of that Apple account, you will not be able to see the code and therefore you will not be able to use these services.
Two-step verification is a security layer added to the name (the email address) and the password. If you activate it, in addition to your email and password you will have to enter that numeric code that you send to the iPhone.
Probably this layer added security suppose to take some more time to use your services but seeing the times, with continuous attacks to get into other people's accounts, it is a good idea to use the verification in two steps.