The CIRCLE is an ancient and universal symbol of unity, wholeness, infinity.  Gnostic traditions linked the unbroken circle to the "world serpent" forming a circle as it eats its own tail.  The circle symbol meaning is universal, sacred and divine.  It represents the infinite nature of energy, and the inclusivity of the universe.  

Our ancient kin observed a circular aspect to the cycles of time, specifically in the movements of the seasons, the integration of energy and matter - spirit and man, combining for the purpose of greater spiritual understanding and evolution. 

Albert Einstein spoke indirectly of it when he said that energy cannot be created or destroyed; it merely changes form.  

Most often when one talks about the "Circle of Life", it indicates that everything in existence participates in this great cycle, including Earth itself.

It is also given as a mark of friendship and respect, for guy or gal to wear or give.