Permissions are used in every forum, permissions are a way for the Board Administrator to keep a handle on things in the forum. Some examples are, the admin makes a Group of Moderators and makes them mod of certain forums, and they can edit your post. Maybe an Webmaster makes you Admin of the forum, this gives you the permission to do all a Webmaster does. To access and change the permissions in a PHPBB forum for groups is
1. Under Group Admin click Permissions and choose a group you have made
2. Then you can see all your Forums and you can choose if that Group is Mod of the chosen Forum
3. Click submit and you are done !
Now permissions are not only good for groups, but they are also good for default forums like "Staff Forum", in the staff forum's permissions you should (if in simple mode) set it as Moderators Hidden that way only Mods and Admins can see that forum, if you want to set it in Advanced Mode I suggest you set most as MOD or PRIV. If you have an "Announcement" forum you can set the permissions (if in simple mode) set it as Moderators, if you want to set it up in Advanced Mode I suggest you put View and Read as ALL; Post, Delete, Sticky, Announce and Poll create as ADMIN; and Reply, Edit and Vote as REG.
Most people know about permissions, this is a little tip to the New Admins ;)