I'm new to this software, but I would put something meaningful in the "Board description" under "Settings". Also under "Settings" you will find "Announcements. This has two parts to it, heading and message. You can use one part for board description and the other for announcements or whatever you want. I looked with a text browser and both "Board description" and "Announcements" are at the top, so Google will pick it up right away. Google will also see the categories and forums on the index page. I would guess that the titles of those would be about as good for SEO as it gets.
Anyway, those are just my crazy ideas. I guess the very best way would be to add a proper meta tag. Perhaps in header.php, but that's getting way over my head at this point.