Fresh off the soundtrack to that shitty monster-hunting film The Mortal Instruments: City of Bones, "Heart by Heart" is the next single in Demi's line of cardiology-themed songs (after "Give Your Heart a Break" and "Heart Attack", of course).
And since musically Demi can't really do any wrong, it's no surprise that this song is a smash. It's all about finding your soul mate and knowing somebody's "heart by heart." Yeah, it doesn't make much sense. But we love her anyway!