So that someone doesn't register on the site just to spam message boards. To activate your email follow the link on the pop up, or go here:
Account Activation
The owner of the Group can make their Group discussion forum visible to everyone or just to group members. The Group Owner can also decide who can create and edit new threads: Everyone, Group Members only, or Administrators.
A thread can be edited to make it open (so users can post to it); closed (so users cannot post to it); sticky (so the thread always stays at the top of the forum); or normal (where the thread moves down the forum as its popularity drops).
The Group Admin can make new threads and edit current ones. They can also edit other users' posts, delete other users' posts and edit threads; making them open, closed or sticky.
If allowed by the Group Owner, non-members can post to a thread and edit their thread.
Group Members can post to a thread and edit their posts, if the Group Owner has allowed this in the Group settings.
The Forum type can be changed to Public/Private. In the Private setting, only Group Members can see the Discussion forum. The rights can also be set as Admin/Members/Everyone - this designates who can post to threads and start new threads.
If you post 3 messages really quickly, then you get the pop-up and are blocked for 10 minutes from adding more posts.
The thread creator can, by clicking on "Edit thread settings" when in the thread.