When you Enable 2-Step Verification on Apple ID it means you put one more layer of security to your Apple account. This 2-step verification will verify your identity using your device before some one try to change account information, make purchases on iTunes or App Store. Here how this works, you will type your Apple ID and password to sign in, Apple will send a verification code to your device and you use this code to complete the sign in. In case if you forget your password or lose your device, a recovery key is also provided to recover your account.
See also: Change your Apple ID
Apple’s 2-step verification option is not available in all countries yet. To see if its available in your country follow the guide below.
Enable 2-Step Verification on Apple Account
Go to Apple ID page and click on Manage your Apple ID button
Sign in to your account using Apple ID and Password
Click on Password and Security option. Type the answer of security questions if you set those earlier in your account, other wise you will see the next screen.
Here you will find Two-step verification heading click the Get started… link.
On this steps read this important text about two-step verification, Click continue button.
Simple and more secure, Click Continue again.
Some important things to remember, Click Get Started button.
For security purpose Apple makes sure no one else is setting up 2-step verification on your account and you have to wait for 3 days. After given time you will receive an email with remaining steps to enable two-step verification. Add it to your calendar for alert or just click Done.