In European culture generally, the "heart" is employed as a metaphor for the innermost essence of something, the seat of its vitality. For instance, heartwood is the wood which is closest to the center of the tree, and Conrad's Heart of Darkness is the center of the Congo, the place where both literal and moral dark is deepest.
Only slightly more narrowly, the heart is seen as the seat of emotions and desires, opposed to the brain, seen as the seat of reason and intellect. The French philosopher Descartes, for instance, famously said that "The heart has its reasons which reason does not know".
This is by no means a universal symbolism. Biblical Hebrew, for instance, saw the heart as the seat of will, which has led to considerable misunderstanding of Biblical metaphor. For instance, Europeans who read that Pharaoh "hardened his heart" understand this to mean that he became hostile and merciless toward the Hebrews, when in fact it means something more like he stopped waffling and strengthened his resolve.