The forum is the place to ask questions, but it's also a place where you can meet new people on the wiki for example.
How to enable it
Go to the page "Special:WikiFeatures" on your wiki and turn it on. If you don't want it any longer you can also turn it off there.
Forum Policies / FAQ
If you want to edit the forum policies / FAQ you have to go to the forum. On the right top you see the button of this, click on it and there will show up a new screen. On the right bottom you'll see "Edit", click on it to edit it.
To manage your boards you only have to click on "Manage Boards" on the right bottom of the forum. You can add, merge and remove boards here and you can change the order of the boards.
For an example of a forum you can visit the forum of this wiki here.