When you look at a thread, you'll find BOOKMARKS among the options at the top of a post. If you click on it, you will have added that thread to your BOOKMARKS, which you can view at any time.
In the PROFILE section at the top of the screen, you'll see a new category called BOOKMARKS. That is where they will be stored. It will be a convenient way to avoid having to search through pages of threads to find one that you remember seeing but you aren't sure when that was.