Mobile phones are everywhere. They are ubiquitous and Google felt to do something innovative and introduced a new security mechanism 2-Step Verification using mobile phones that adds an extra layer of security to users’ Google accounts by requiring them to enter a verification code in addition to their username and password, when signing in to their account.

When an user signs in his/her account, he/she has to enter verifications code after verifying his/her username and password which is delivered to his/her mobile phone every time. However user can set expiration of code for 30 days on a particular computer.
Here are some useful links that help you to get started with 2-Step verification:
How it works | Setup | Signing in with verification codes | Signing in using application-specific passwords | Turning off 2-step verification | Common issues