From what I understand, the Basic > Forum Description field in the Administration are is for the meta tag used by engines to get a short description of your site. The problem is that, even if I fill it up with something, the meta tag is still empty in the source.
I don't know if this is some cache related issue because I changed it yesterday and is still empty. Search engines go nuts and show some "This site is best viewed in a modern browser with JavaScript enabled" in the description, instead of what I have chosen