Short answers, pulled from the story.
The god Set tore out one of Horus's eyes during their violent struggle for kingship after Osiris died. Sometimes Set trampled the eye or ate it entirely before Thoth restored it using specific plants and minerals.
Ancient Egyptians buried Wedjat eye amulets with dead bodies as offerings to sustain gods beyond death. Until the New Kingdom began around 1550 BC these amulets sat on the chest of the deceased before moving to cover incisions made during mummification.
The Hearst papyrus describes a doctor as Thoth the physician of the Eye of Horus while Papyrus Leiden I 348 assigned each body part to a specific deity for protection. These spells invoked divine power to protect against actual eye diseases by drawing the wedjat eye on linen or papyrus.
Artisans crafted them from faience glass gold and lapis lazuli with some designs including uraei cobras at the front or bird tail feathers at the rear. The Third Intermediate Period saw complex versions with multiple small animal figures inserted into gaps.
Georg Möller proposed that the inner corner stood for one half while the pupil equaled one quarter and the eyebrow marked one eighth. He stated the outer corner represented one sixteenth a curling line signified one thirty-second and the cheek mark one sixty-fourth.