The heart is the infinite, divine part of you (your spirit). Your spirit knows who you are and what you're here to do. The heart is your inner knowing, the part of you that transcends mere emotions or intellect and sees and knows the path you need to follow.
Self is the source of all courage and self is the source of all weakness. Self is the source of all joy and self is the source of all sorrows. That very self can make this world a beautiful place and that very self can also make this world a miserable place. The only thing that distinguishes the one from the other, is your intention.