This add-on integrates HybridAuth library into your store and allows users to log in and register using various social network services such as Facebook, Twitter, Google, etc.
The add-on should be installed and active in your store right out of the box. If it's not, just go to Add-ons → Manage add-ons and install it.
You can add social login providers on the Website → Social login page, by clicking the Add provider button (the "+" button in the upper right corner of the page).
In the popup dialog, you should then pick a service (e.g. Facebook), and enter the ID of an application and a secret key. Now these values you get directly from the service by creating an app there. The particular routine depends on the service, so we won't cover tis part here, but it should be easy.