Concentration is not thinking of one thing. On the contrary, it is excluding all thoughts, since all thoughts obstruct the sense of one's true being. All efforts are to be directed simply to removing the veil of ignorance. Concentrating the mind solely on the Self will lead to happiness or bliss. Drawing in the thoughts, restraining them and preventing them from straying outwards is called detachment (vairagya). Fixing them in the Self is spiritual practice (sadhana). Concentrating on the heart is the same as concentrating on the Self. Heart is another name for Self.
Ramana MaharshiThe seat of Realization is within and the seeker cannot find it as an object outside him. That seat is bliss and is the core of all beings. Hence it is called the Heart.
Ramana MaharshiHappiness is your real nature. You identify with yourself with the body and mind, feel it's limitations, and suffer. Realize your true self in order to open the store of happiness. That true self is the reality, the Supreme Truth, which is the self of all the world you now see, the self of all the selves, the One real, the Supreme, the Eternal self - as distinct from the ego or the bodily idea for the self.
Ramana Maharshi