The 2FAS plugin adds the second factor to the login process. After you enter your username and password, you must enter a specific token provided by a mobile application or another source (e.g. text message). It is called two-factor authentication, and it increases the protection level of your website. You can read more about two-factor authentication here.
After you activate the 2FAS plugin, it does not exploit its full potential yet. To provide all features registration is needed.
Registration is needed because the 2FAS plugin communicates with the powerful 2FAS API. That gives an opportunity to make authentications, send text messages, make automated voice calls and many more.
If that has not convinced you yet, you can still check 2FAS Light, a plugin which does not require any registration, but it offers fewer features than the 2FAS plugin.