Add a word to the title of every new topic in a certain forum folder?

[kh99]

The description is the text of the first post, and the value for the description tag is that text with quotes and bbcode removed, censored, then cut down to 500 chars max. That's all done at the time the thread is displayed so if you want to modify the description meta tag on the showthread page then you'd need a plugin using a hook like showthread_complete, and you can modify the string in $thread['meta_description'].

Edit: Oops. Badshah beat me to it.