This symbol represents the three phases of the moon—waxing, full, and waning.  The moon phases are deeply symbolic of the cycles of life, and also of the Goddess in Her three aspects of Maiden, Mother, and Crone. 

The first (left-hand) crescent stands for the waxing moon, and all its associations of new beginnings, new life, restoration and healing. The circle stands for the full moon, a time for passion, fertility, and powerful magick. The last (right-hand) crescent stands for the waning moon, a time to do banishing magic and allow the cycle to come to an end.