The optional security feature lets you require verification of your identity using one of your devices before you make changes to your account, make a purchase, or get Apple support.
Your Apple ID is the key to many important things you do with Apple, such as purchasing from the iTunes and App Stores, keeping personal information up-to-date across your devices with iCloud, and locating, locking, or wiping your devices. Two-step verification is a feature you can use to keep your Apple ID as secure as possible.
How Does It Work?
When you set up two-step verification, you register one or more trusted devices. A trusted device is a device you control that can receive 4-digit verification codes using either Find My iPhone notifications or SMS to verify your identity.
Then, any time you sign in to manage your Apple ID at My Apple ID or make an iTunes, App Store, or iBookstore purchase from a new device, you will need to enter both your password and a 4-digit verification code as shown below.
Enter your Apple Id, receive a verification code, enter your code to verify your identity and finish signing in. After you sign in, you can manage your account or make purchases as usual. Without both your password and the verification code, access to your account will be denied.
You will also get a 14-digit Recovery Key for you to print and keep in a safe place. You will use your Recovery Key to regain access to your account if you ever lose access to your devices or forget your password.
Enable Two-Step Verification:
You can add two-step verification to your account by visiting the link below. Select "Manage your Apple ID", sign in, select "Password and Security", then click "Get Started" under "Two-Step Verification".
Initially, two-step verification is being offered in the U.S., UK, Australia, Ireland, and New Zealand. Additional countries will be added over time.