From ThelemapediaHeru (Horus), wadjet, utchat, or sometimes the Eye of Ra.
According to Egyptian mythology, during the contendings of Heru and Suti (Set/Seth) for Osiris' throne, Suti ripped out one Heru's eyes. The damage was later healed, depending on the myth, either by Het-Hert (Hathor) by milking a gazelle and putting the milk into the empty socket, or by Tehuti (Thoth) through his magic. Some myths state that Heru then gave the Eye to Osiris so that he may rule the underworld.
The healing of Heru's eye gave rise to the symbol being used as a healing and protective amulet. It was believed that placing it over an injury would magically heal it.
From the same myth, some Egyptians also believed that Suti tore the Eye up into sixty-four pieces. Fractions were assigned to each part of the Eye torn by Suti, which only added up to 63/64. Tehuti restored the eye, compensating for the missing part with his magic, and established it as a symbol of the restoration of natural order.
Heru's right and left eye represent the sun and moon, respectively. Thus, when the udjat is shown as a left eye it is linked with Tehuti, and with Ra when as a right eye. Together, the two eyes represent the entire universe. There have been suggestions that the two eyes are of a dualist nature, acting much like the yin-yang symbol of Taoism; the right eye representing masculine energies, and the left representing female energies.