Facebook Login permits users to sign in to 3rd party sites using their Facebook account. It's handy, as it allows to create accounts instantly with a few clicks skipping the registration process, no need to fill in and submit any forms. If technical mumbo-jumbo doesn't scare you, you may read about this technology here.
In a nutshell, it adds Facebook Login button to your site, clicking on which users can sign in right away. Setting it up requires no programming skills from your part, you only need to setup Facebook Application.