What is 2-step verification?
In addition to usual login method, a unique code will be sent to your mobile phone which is registered with Google account (This code will be used to enter Gmail interface). Consider if someone knows your credentials, but that person will not be able to login without the unique code, which will be sent to your mobile phone.
To turn ON 2-step verification, sign-in to Google account (https://www.google.com/settings/) and move to security (https://www.google.com/settings/security).
You can find 2-step verification as shown.
Enter the phone number,
Enter the verification code,
Confirm 2-step verification.
2-step verification has been confirmed.
When you login with your credentials, it will now ask you to enter the unique code as shown below.
Note: Consider getting the backup codes, this will be used if your mobile is unavailable.