2-Step Verification in Gmail

Every time someone logs in to your gmail account, google account sends you a verification code via SMS or phone. So the person who wants to login to your google account without entering the verification code will be rejected and can not log into your account. To enable this 2-step verification feature, you must first add an active mobile number (your own number, not others). If you have not added it yet, you can read the tutorial here.

After that, consider the steps below for how to enable 2 step verification google account.

1. Login to your google account via the following link.
2. Then select Sign-in & security.
3. To enable 2-step verification, click 2-Step Verification.
4. If you have never enabled 2-step verification, Please select GET STARTED to continue.
5. For the purposes of verification, enter your google password. Then select NEXT.
6. Select the phone number you want to use for 2-step verification. In the following article, I just added 1 phone number only in my google account. So the phone number will be automatically selected as shown below. In the How do you want to get codes field, select Text message only. Text message (verification code sent via SMS), while Phone call (verification code will be notified via phone call). Next select NEXT.
7. Google will send you a verification code via SMS. Enter the code you received, then select NEXT.
8. If the verification code you entered is correct, you will find the page as shown below. Then select TURN ON and 2 step verification for your google account is active.

So every time someone else logs in to your Google account from an unknown device, this 2-step verification page should appear and even if the person knows your email address and password, it will be difficult to enter the verification code first if you want to login.

Source : https://en.wikipedia.org/wiki/Stephen_Covey