Lyric discussion by Asshattery 

Cover art for What You Know lyrics by Two Door Cinema Club

To me it seems like he is talking about this girl he's always wanted and she has always said no, or didn't feel the same way, and now she finally says yes for some reason or another, maybe its cause he's getting more famous. And he still wants to be with her, but he feels her to be just using him "Just" to have a boyfriend. That's what I think, but yeah.