every human being has a heart, their heart is filled with various traits, there is anger, revenge, forgiveness, affection and others.
for those who have trained their heart with good deeds, by means of meditation (khulwah), to practice their patience of heart so that to the stage of Sufi is those who walk the path of goodness and succeed in it.
while hearts still filled with anger and revenge will even further damage the body and mind, which will ultimately destroy the human life itself.
the Sufis equate the heart like the ocean, the deepest sea, the endless sea, because the heart keeps a variety of mysteries, no one can know it, no tool whatsoever can assure the content of one's heart, like the sea, the sea is full of treasures , the ocean is full of riches that can not be reached by anybody, whoever he is, searching for something in the ocean is like looking for a needle in a haystack.
the heart is the source of all secrets, I do not know explain in the views of other religious beliefs, but I believe all religions believe the heart is the source of everything,
my friend @reddust has explained his heart and his tinnaannya in the Buddhist view, I give a comparison in Sufi teachings, there are similarities in addition to having a difference from the base and the end, but basically the same, while the path that conveys the two is also different, tasauf through the way of khulwah and tafakkur, while the Buddhist way is called meditation.
The purpose of both is to achieve clarity of mind so that it comes to the praiseworthy nature,
in the end the heart is the ocean, only a few human beings are deeply immersed in its sea of hearts.