If it were not a closed group, then everyone can see all of the post. This way, you can find the group, see who's a member, but you can't post or see post. You have to ask to join, and get approved.
Public group Open to anyone at any time.
Closed group Open to see that the group can be found and exist, but need to ask to me a member.
Private group. Privet means you can't even see it. Someone from inside the group has to invite you. That to me, was a bit too Fraternal.