This sacred symbol is a Mayan timekeeping calendar of 260 days.  One theory of the purpose of this calendar came from mathematical operations based on the numbers thirteen and twenty, which were important numbers to the Maya.  The number twenty was the basis of the Maya counting system, taken from the total number of human digits.  Thirteen symbolized the number of levels in the Upperworld where the gods lived.  The number 260 is significant because it adds up to the number of "joints" in the human body (ankles, knees, hips, shoulders, elbows, wrists, and neck). The numbers multiplied together equal 260. 
At the center of the Tzolkin is the symbol Hunab-ku or The Tree of Life and The Holographic Universes.  Hunab-Ku means “The One God.”  The symbol reaches out to eight directions, with four main directions and another four in between them. Resembling the Yin and the Yang symbol of Taoism, the Hunab-Ku is also about the polarity of male-female or light-dark energies.  Hunab-Ku is thought to be at the center of the universe, and it is the source of consciousness and everything else.