When a camera flash is fired directly into a person's face, the light can reflect back from the subject's retina, giving a red spot in the centre of the eye.
This is not as common these days as it used to be, mainly because many cameras have a facility that can avoid it.

If the person is not looking directly at the camera or if the flash is little distance from the camera, then 'red eye' will not occur.

The Red Eye Tool can remove it instantly.

1. Open an image with the offending red-eye.

2. Pressing Ctrl + = / Cmd + = enough times to make the image 200% size as shown in the top of the image frame or in the bottom left corner.

3. Hold the space bar and when the cursor changes to the hand symbol,
move the image around so that the eyes are on the screen.

4. Choose the Red Eye Tool in the toolbox. It is with the Spot Healing Brush.

5. The default settings in the options bar are at 50%. Start with these values.

6. The cursor is a cross; click on the red dot in one eye.
The red is filled in with black. The black circle will have a feathered edge.

Usually the default settings will be fine, but if the feathered edge leaves a hint of red, then Pupil Size can be increased.
Undo the last move with Ctrl + Z / Cmd + Z and reset Pupil Size to 60%.

To improve the blackness, the strength of the dark can be increased as well, by increasing the Darken Amount value.