Generally stands for 'Yea I've noticed that'. Most people use this phrase when someone makes a sarcastic any typically untrue comment. Rather than accepting the sarcasm, this phrase can be used to deflect the sarcasm back at the original commenter.
Random Female A: I stopped paying attention to you a long time ago. I hate video games
Random Male B: That's funny, because did you know the average female gamer is ridiculously hott?
Random Female A: YINT