It's the song in your heart that wants you to help make yourself a better person, and to help other people do the same. Everybody has one.
A heartsong doesn't have to be a song in your heart. It doesn't have to be talking about love and peace. It can just be your message. It can be your feeling. Some people might even call it a conscience, even though that's not really what it is. It's your message, what you feel like you need to do.