"Hint, hint" is what you say when you're trying to get someone to do something or realise what you're asking them for, when it would be rude or inappropriate to ask them to do it directly.
It is said humorously to make it a bit more obvious that you're giving hints as to what you'd like them to.
Brother: My friend is coming round tonight, and we're hoping to have a little bit of time alone together, hint hint, if you get my drift, hint hint.
Sister: OK OK, I get it, I'll get out of here.
Girl: Mmm, I think one of those engagement rings would look really good on me, don't ya think (hint hint)?
Guy: Hey, look at that... thing over there!