What does the word heart mean in the Bible?

In both Hebrew and Greek culture, the heart was thought of more as a symbol of ultimate purpose and motivation. Emotional concepts, like affection, were associated more with gastrointestinal organs. Hebrew symbolism used the body as an analogy of the whole person. The strength of a person was their outward behavior - what they do WITH their body. The conscious mind, in that culture did not as easily separate thought, emotion and volition like we do. They recognized the conscious mind was engaged in thoughts, feelings and intentions but they tended to see these as different facets of the conscious mind. The heart is the organ more at the core of the body. That culture used the heart as a symbol of the inner man, the deepest thoughts, the deepest feelings, the deepest intentions. So deep at times they were incomprehensible, and in some ways, unknowable and difficult to observe - known only by God.
So the heart is, in that language, a word that represents the core or ultimate intention, ultimate value, ultimate motivation. These were also analogous to concentric circles. The inner man is center-most. This is the heart. Surrounding the heart is a larger circle called the mind. The mind included all conscience thoughts, decisions and feelings. The outermost circle was the body where all the behavior took place. Spirituality starts at the center and works its way out, hence the admonishment to love God with all your heart (the very core of your being), all your mind (the most accessible part of your conscious life) and all your strength (the behavior that flows from the heart and mind).
Early Greek was not too different, especially the dialect of Koine Greek spoken in Israel and in which much of the New Testament was written. So in the bible, the word heart included all the facets of conscious process of intent, thought and feeling but at a level so deep it is ultimate and less accessible. It does not mean "emotions" they way we use the word today.