"Just not" expresses the feeling that, even though you want something to be true, it's not true in reality. There's a phrase that was made famous by the TV show "Sex and the City", and later used as the title for a movie:
Being "into" someone means that you like them a lot. This sentence was told to a woman who was worrying about why a man who she'd gone on a date with hadn't called her back. When she heard "He's just not that into you," it convinced her to stop worrying about it.