A third-party Login allows users to easily and quickly register in your community through the "Facebook", "Google", "Twitter" and "GitHub" portals. The advantage of this is that the profile data is taken over by the respective provider and the registration is done by a few clicks. Note that for security reasons the login is not supported for administrators via third parties!
Common to all vendors is that you have to create an "application" for the respective vendor, and get an ID and secret key, which must be entered into the settings (Administration > Configuration > Options > User > Registration) of your community:
The individual vendors