Horus defeated the evil Seth, his brother in battle avenging his Father’s death, Osiris. In battle while fighting his brother Seth, Horus lost his left eye, which was later restored by the God Thoth and from that healing he received magical powers of sight and far seeing visions. The eye became a sacred symbol of protection, the Udjat Eye, or the Eye of Horus which is associated to the left side, the heart side and is the left eye is the Eye of Horus.
The sacred Eye of Horus was perhaps the most worn amulet, second only to the scarab. Besides being protective, it repels the evil eye, and was used by most people in Ancient Egypt, pharaohs, as well as the commoners.
It is a common symbol of protection even today worn by most followers of the Old Religion.