Literally, it's talking about sounds. Bass notes are low, while treble notes are high. More important is the metaphorical meaning. To quote the artist from an interview: /Interviewer:So you’re all about that bass, but no treble. Metaphors, perhaps? Meghan Trainor: It’s like thickness and thin: bass is big and treble is the high, thin stuff. Bass is like booty, but treble isn’t boobs. It’s just a joke about thick and thin./ Here's the full interview, if you want to read it. http://www.theguardian.com/music/2014/oct/02/meghan-trainor-all-about-that-bass-interview-im-getting-flak