From a Guardian interview: Q: So you’re all about that bass, but no treble. Metaphors, perhaps? MT: 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. (http://www.theguardian.com/music/2014/oct/02/meghan-trainor-all-about-that-bass-interview-im-getting-flak)

"Bass" is the bottom part of a musical arrangement, and is also a homophone for "base," which means bottom. The video here seems to make it clear that she is using it to mean butt, buttocks, bottom. "It's all about X" means "X is the most important thing."

Hiya! It's just the debut single of Meghan Trainor and doesn't mean anything specific really; it's not an idiom or something. However, the bass as bass is a musical term/word and has to do with the sound of a music piece.