The word heart refers to a hollow muscle that pumps blood throughout most species' bodies. The word can also be used to refer to the geographical center of a certain area. This is because the "heart", is located in the chest, or the "center" of the human body.
Because of this, the heart is often said to be the source of thought, reason and emotion. Technically the brain is what gives humans that function but this refers to the heart in a more metaphysical sense.