The heart of something is its core, the most central part of that thing. The heart or core of regenerate man is the place between his body, soul, and spirit, that intersecting and connecting point where they communicate with each other.

"A plan in the heart of a man is like deep water,
But a man of understanding draws it out." (Proverbs 20:5)

The heart of man is the place containing his deepest thoughts and intents.

"For the word of God is living and active and sharper than any two-edged sword, and piercing as far as the division of soul and spirit, of both joints and marrow, and able to judge the thoughts and intentions of the heart." (Hebrews 4:12)

The heart of man is his true being, his innermost being, who he really is.

"As in water face reflects face,
So the heart of man reflects man." (Proverbs 27:19)