Wikipedia defines Easter Eggs (in software) as:
An Easter egg is an intentional inside joke, hidden message, or feature in an interactive work such as a computer program, video game or DVD menu screen. The name has been said to evoke the idea of a traditional Easter egg hunt.
Easter Eggs (done right) make users smile. It can be something as simple as an unexpected shortcut for a common task. It might be a a hidden menu. Maybe it’s just a funny pop-up box that happens on certain actions.
Easter Eggs form a bond between a user and a system. It’s not just about performing a function, it’s about making your tasks fun. They don’t have to be obvious or even something that happens occasionally. It may be just the way that language is used. If you make software with a sense of humor, you will find you have more engaged users.