What does it mean when it says "closed group" on Facebook?

[Thomas]

[Comicbook Reader]

It means you have to ask to join the group. The people who run the it don't want to be bothered with trolls.

[Fiona]

it means that you have to be asked by a member of the group to join it

[?]

It means that you want to close if forever you will not use it agan

[Jennasaurus]

It means that you need to apply to join the group, which is easy because there is always a button on the page to ask them. When (if)) they accept the request, you will get a notification via email. The reason they do this is to keep unwanted people outside of the group.