We assume you started in a particular forum, had a list of topics for that forum, and clicked to go to one of the topics. How do you get back?
You can always use the browser's Back button. Within the community UI...
If you are in a topic, look to the top left of the window. Click the name of the forum, and you're back on the Content page.
If you are in a subforum content page (like one of the individual support forums), then in the banner, again toward the top left, you'll see the forum name, and under that the name of the parent forum. Click that, and you're back to the parent forum's content page.
You can, of course, get to the Overview page for any community by picking the community from the top Communities menu.
See also: Basic Navigation