Pentagram

A pentagram, or pentacle, is a symbol used in Wicca and witchcraft.

Usage
The girls used a pentagram in several spells, but it's unknown what they used it for. It's presumed that it was used to enhance their spells.

Origin
A pentacle (or pantacle in Thelema) is an amulet used in magical evocation, generally made of parchment, paper or metal (although it can be of other materials), on which the symbol of the spirit or energy being evoked is drawn. It is often worn around the neck, or placed within the triangle of evocation. Protective symbols may also be included(sometimes on the reverse), a common one being the five-point form of the Seal of Solomon, called a pentacle of Solomon or pentangle of Solomon. Many varieties of pentacle can be found in the grimoires of Solomonic Magic; they are also used in some neopagan magical traditions, such as Wicca, alongside other magical tools.

Modern Witchcraft
Some modern witches or neo-pagans believe that the pentagram represents the five elements often used in magic, with each of the star's points representing a particular element. These elements are earth, air, fire, water and spirit.

Trivia

 * Some people mistakenly associate the pentagram with Satanism or devil worship. In fact, the original pentagram symbol has nothing to do with Satan, although the inverted pentagram (an upside-down pentagram) is used as a symbol by the modern Church of Satanism (although their adherents do not worship the Judeo-Christian Satan).
 * Pentagrams are also used by witches in other fictional works, including Buffy the Vampire Slayer, The Vampire Diaries and Charmed.