PhoneFactor is a plugin that provides an additional layer of security to your WordPress site. Upon installing the plugin, you will enter an authorized number which will be used to confirm login attempts are legitimate. When you log in to your WordPress blog with PhoneFactor enabled, you receive a call to the authorized number and confirm your login attempt by pressing #. When you do so, you are logged in. If you do not, then the login attempt fails.
Essentially, this creates a two-factor security system for accessing your website, meaning someone needs to get your username, password and phone in order to log in to your WordPress site as you.