Usergroups are the separate groups that all members are in, such as registered users, Mods, Admins, etc. There are several registered members usergroups. The first one is what all members become when they activate their accounts. The second one, after they reach a post count of 40, to access the B/S/T forum.
Some of the movement from one usergroup to the next is by an automatic promotion. Before today, the promotions added the member into a secondary usergroup. After today, the promotion will make it the primary usergroup. That might not mean alot to most members, but you may see some changes in your privileges.
If you find there are problems with your acccount, please let me know.