Red-eye is an effect that often occurs in flash photography when the camera flash reflects the blood in the back of the eye. As a result, the people within the photo appear to have red eyes.
This effect is especially noticeable when taking photos in dark rooms. The pupils naturally expand in the dark, allowing more light to enter the eye and reflect to the camera.
Additionally, the less melanin a person has in their eye, the stronger the red-eye effect. For example, people with light skin and blue eyes have low melanin levels, allowing the blood in the back of their eyes to reflect more strongly with the camera flash.