The Social Login allows to the user to connect and login to your site by using a social site account. Actually 5 social sites are supported: Facebook, Google, Microsoft Live, Twitter and LinkedIn. In the future we are planning more.
To reach the Social settings, you need to login to your Joomla! administrator page.
After successful login, go to the Components/Improved AJAX Login & Register/Social Settings. You will see a list from the available social types. To edit it, click on the name, and the editor will load. All of the types has a little tutorial on the right, so this part won't be detailed here. Just follow the few steps, and you will succeded. Note that you need to read the Social Parameters article, and allow the social login parameter in the module manager.