To move widdershins is to move in a counterclockwise direction. This archaic term is not often used in Wiccan ceremonies; if it is used, it's typically applied in magic related to banishing or destruction. The Wiccan Rede includes a line about "Widdershins go when the moon doth wane," which means that banishing magic should be done during the waning moon and not the waxing phase. The opposite of widdershins is deosil, which means clockwise.
In a ritual aimed at eliminating a bad habit, Willow moved in a widdershins direction around the altar.