Apple's photo editor has red eyes. What is that? What is this for?

The word "red eye" is actually aimed at the photographer. When the flash illuminates the human eye, the pupil will enlarge to let more light pass through, and the blood vessels in the retina will turn red in the photo. If the subject's eyes are dark, the degree of red eye phenomenon is determined by the depth of pigment, so sometimes the red eye phenomenon is not particularly obvious.