The heart is a powerful symbolic representation of whatever fragment remains in a monster’s soul. Some monsters are so cut off from what it means to be human, that they have to keep the heart hidden far away. Like a true name, the heart is the same as the soul. Its location is a bitterly guarded secret, which can only be taken from the end by trickery or force.
Possession of the hidden heart can give the discoverer power over the end. However, it is a twisted dangerous thing. It may radiate such an aura of evil, and one must take care against possession or being overcome.
The hidden, or secret, heart need not be an actual beating heart, but it may refer to any relic or place where a monster’s soul is bound.