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.

A save function would be great if implemented. NGL, I just send things I want to save to Google Keep.