The heart is a part vital to the existence of any living thing. Just as people have hearts, so do worlds.
If an individual, be they good or evil, gives into the darkness of their heart, they will become a Heartless, a being of pure darkness who seek out other hearts. Usually, the body fades away. However, if the heart was strong, then the empty shell that is left behind will become a Nobody, and then seek to reunite with their hearts.
Memories are also tied with the heart.
It should be noted that the 'heart' is one of the three things that make up existence of a living being. The other two are the 'body' and the 'soul.'
