Each forum has a "URL Portion" setting which is blank by default. That URL Portion effectively becomes the ID of the forum.
e.g. Our Announcements forum has a URL of https://xenforo.com/community/forums/announcements/
Our URL Portion for that forum is "announcements".
Although this may seem like a good idea, do bear in mind that changing the URL Portion (or removing it) will effectively cause all links pointing at it to be broken. (You would have to handle that with manual redirects or Route Filters if that happened).