Google has introduced its opt-in security feature called 2-step verification to all users in an effort to make their accounts more secure.
The 2-step verification includes your password and code you get via your phone. The Google Blog provides more details.
“Take your time to carefully set up 2-step verification—we expect it may take up to 15 minutes to enroll. A user-friendly set-up wizard will guide you through the process, including setting up a backup phone and creating backup codes in case you lose access to your primary phone.”
“Once you enable 2-step verification, you'll see an extra page that prompts you for a code when you sign in to your account. After entering your password, Google will call you with the code, send you an SMS message or give you the choice to generate the code for yourself using a mobile application on your Android, BlackBerry or iPhone device.”