The One Time Passwords (OTP) are the second layer of security for your account.
They allow you to have an extra level of security when performing critical actions for your account like logging in.
You can link your google account, or set up one-time passwords. To set these two up follow this guide.
Console Console > Account > settings > Scroll down to Linked services & One time passwords
You can log in using your Gmail, Twitter, or Apple accounts, just click "Add +" and follow the setup instructions.
One time passwords (OTPs):