Authenticator App
First tap the plus to add a new 2 factor code.
Codes are provided by services directly (EX: Amazon, Facebook, etc). You find them in their account security section. Scan the code to add it to the app.
Once the QR code is scanned you get a "2 factor code" or "one time password".
You can now use the code when logging in to your service provider Website or App in the relevant field.