Two-factor authentication means using another means, besides just a password, to login to your favorite website. These “tokens” used to be little RSA plastic devices, mailed mostly by banks. Today, you can use your smartphone for that task!
Once you enable tw0-factor authentication, you login to the website, just like you normally would, but then are prompted for a 6-digit code. This code is displayed on your phone, either by an app or text message. This means you must have the phone with you, preventing any hacking from taking place.
The following are some common websites that allow you to use two-factor (or multi-factor) authentication:
Some apps for managing these on your smartphone: