2 Step Verification on Amazon.
Amazon's Two-Step Verification is a security feature that asks you to enter a unique code besides your password on computers and devices that you haven't designated as trusted.
Go to “Advanced Security Settings.”
Click “Get Started.”
Add your primary phone number (this phone must be able to receive SMS messages) or download and configure an authenticator app and click “Send code.”
Enter the code that’s sent to your phone number or generated through the authenticator app and click “Verify code and continue.”
Do either of the following;
Add a back-up phone number and decide on delivery format (text message or voice call).
Download and configure an authenticator app. This will allow you to generate security codes when you're unable to receive text messages.
You won't be able to turn on Two-Step Verification without adding a back-up phone number.
For instructions on how to disable Two-Step Verification click, here.