An Easter Egg is an intentional hidden message or feature within a DVD, CD, video or computer program in which when found and triggered can give access to bonus features or shows something that is not mentioned in the main list of contents.
Some Doctor Who discs contain hidden features that are more or less easy to find. These features are usually so small and insignificant that they would not really be appropriate for inclusion on the main disc menu. However, disc producers sometimes like to sneak these little gems onto the disc in "hidden" places.