I'm a ghost and you know this
That's why we broke up in the first place
He’s a ghost meaning he’s not around anymore and that he’s a travelin' man, which is why they broke up in the first place (a theme Drake, Kanye West, and Hank Williams have also expressed in song). This also makes him an emotional ghost. He also flips an accusation she had of him: she accused him of being a ghost while they were together, and now that he is a ghost, she only wants him back.
To help improve the meaning of these lyrics, visit "Heartbeat" by Childish Gambino Lyrics and leave a comment on the lyrics box