This symbol, sometimes called the Triple Goddess symbol, represents the three phases of the moon -- waxing, full, and waning. According to Robert Graves' The White Goddess, it also represents the three phases of womanhood, in the aspects of Maiden, Mother and Crone, although many scholars have questioned Graves' work.
This symbol is found in many NeoPagan and Wiccan traditions as a symbol of the Goddess. The first crescent represents the waxing phase of the moon -- new beginnings, new life, and rejuvenation. The center circle is symbolic of the full moon, the time when magic is at its most potent and powerful. Finally, the last crescent represents the waning moon -- a time to do banishing magic, and to send things away. The design is popular in jewelry, and is sometimes found with a moonstone set into the center disc for additional power.
Invoke this symbol in rituals such as Drawing Down the Moon, or in workings involving lunar goddesses.