When you upvote a thread, it's counted as a "favorite." E.g. If you look at the list of people who favorited this thread, you'll see me listed because I upvoted it. And if you click on the "Favourites" link in the main menu, you can see all the threads that you upvoted.
Yeah, what FaceBear said, "boosts" are better as a "saved" list. Boosts are unique to Kbin, and currently they determine a thread's popularity, not upvotes as you would think. That's supposed to change soon, according to the instance owner (who is a real nice guy btw).