Security groups define who is able to view and interact with your communities, eGroups and Resource Libraries. Many of your security groups will be determined from your AMS configuration, but Higher Logic includes several "standard' security groups:
Public: when checked, the content is accessible by anyone, whether logged in or not
Authenticated: when checked, the content is accessible by those groups designated in your AMS, but only when logged in
IsMember: when checked, the content is accessible by those that the HL site considers a member. This was configured during your set-up and is also defined by your AMS. It generally takes into account multiple member types, membership integration, receives member benefits flags, grace days, and so on.
HLAdmin: when checked, the content is accessible by those with super-admin rights