Easter eggs are hidden features or messages in a program. They are secretly included by programmers in some software to be a fun surprise for the users who discover them.
Easter eggs are also known as "undocumented features". They can take the form of secret messages, humorous additional features, or in-jokes amongst the development team.
The inclusion of Easter eggs in modern software is typically discouraged, as they can be seen as unprofessional or introducing potential security issues, since they are often not tested like the rest of the code. Most Easter eggs are typically harmless, however.
To see how you can make Easter eggs in Markdown, check out this section of the Markdown Editing guide. ;)
For further reading, see this Wikipedia article on Easter eggs.