Traditionally, painted eggs were used as decoration at Easter. More recently, they have mostly been replaced with chocolate (usually egg or bunny shaped) and hidden for children to find on Easter morning.
In computer terms, 'easter egg' refers to a hidden area or feature that is can be triggered by something obscure enough that it could not be found by trial and error or by accident. For example, there was a certain combination of mouse clicks, button presses and the MS Excel 95 help screen which would pop up a window with a 3D maze. Upon successfully navigating the maze and getting up a narrow ramp (not trivial with the 1st person perspective), you got to a room with paintings on the wall, and the one at the end had all the names of the programmers scrolling by.
Less elaborate references, areas or items are generally easier to find on your own, without necessarily needing a detailed description (such as the Kaa Gent scroll;
Catacombs under Warrior's Guild query...).
Easter egg.
Easter egg (virtual)