@tedtcb,
That's a method to vote up or down an entire thread
- to show you appreciate the thread or are negative about it (a down thumb will make it "invisible", reduced to just a Topic Voted Down (view) in tiny print. Clicking view will let you see it again.
- or as a kind of housekeeping, to clear out those threads you don't want to see again.
Something similar happens with thumbs on individual posts within a thread, but is used more to express appreciation or disapproval. You can also view those by clicking on view.