How do favorite questions work?
I saw a star that appears at the bottom of my question. When I point my mouse on it, I am getting a message This is a favorite question (click again to undo).
How it will be useful to me?
This adds the question to your favourites (or removes it) if you click it; they can be viewed on a tab on your profile. This more or less allows you to find questions you like more easily later on.
Actually, I think that the tooltip text is really off, and confusing. How about something like, "Add/remove question to/from favorites." Just less meh.