When you take pictures using a flash, your subjects can sometimes be afflicted with red eye, an annoying condition in which the iris (the colored part of the eye) glows an eerie red. You can use the Redeye tool to change the eyes to a more natural color, salvaging an otherwise unusable shot.
Note: When viewing the edited photo, increase the magnification so you can closely examine the eyes. If they still contain red, repeat the Redeye Repair.