A plugin can add an authentication method to your app, through OAuth. OAuth (Open Authorization) is an open standard for token-based authentication and authorization on the Internet. It allows an end user's account information to be used by third-party services, such as Facebook, without exposing the user's password. Practically, you'll be able to add a button to your app 'Login with Facebook', and users, instead of typing a password in your app, will authorize Facebook to grant access to your app.