The phrase "he is not that into you" means that someone (a man, in this context) doesn’t have strong romantic or emotional interest in another person. It suggests that if someone is truly interested, their actions will show it clearly—through communication, effort, and consistency. The idea was popularized by the 2009 film He’s Just Not That Into You (based on the book by Greg Behrendt and Liz Tuccillo), which uses different characters’ relationships to highlight this point: when someone wants to be with you, it won’t be confusing or uncertain. Would you like a brief explanation of the main lessons or relationship advice from that book and movie?