Horus was the ancient Egyptian sky god who was usually depicted as a falcon.
His right eye was associated with the sun Ra. The eye symbol represents the marking around the eye of the falcon, including the "teardrop" marking sometimes found below the eye. The mirror image, or left eye, sometimes represented the moon and the god Djehuti (Thoth)
Eye of Horus in hieroglyphs
In one myth, when Set and Horus were fighting for the throne after Osiris's death, Set gouged out Horus' left eye. The eye was restored by either Hathor or Thoth. When Horus' eye recovered, he offered it to his father, Osiris, in hopes of restoring his life.
Hence, the eye of Horus was often used to symbolise sacrifice, healing, and restoration.