Very briefly, this is part of the taoist take on the nature of reality, the "as above so below" principle coded in the binary above-below numerical system of the I Ching. "Heaven above, 32, Earth below, 32" is a statement often encountered in taoist alchemical literature.
The set of 64 hexagrams exhibits a perfect balance:
32 hexagrams are Yang in nature. These are the hexagrams that contain an odd number of solid, yang lines (1, 3, or 5).
32 hexagrams are Yin in nature. These are the hexagrams that contain an even number of broken, yin lines (2, 4, or 6).
The human body, measured in kuns, is 64. A kun is a measurement unit (used, e.g., by competent acupuncturists for locating points) for a particular individual, approximately the width of that individual's thumb. It is of course slightly different for all people, but based on the fractal nature of the body, there will be corresponding differences in the length of every part, and each anatomically normal human body will measure 64 kun. The lower "heaven" part and the upper "earth" part are each 32.