whats with “Cake” in portal the game?

is this some inside joke that i keep hearing about ..its supposed to have something to do with the game “Portal” …so what’s it about?

During the corse of the game you are promised cake as a reward. But then you start finding “The cake is a lie!” scrawled all over walls in hidden rooms. Then you finish all the tests and find that the cake was in fact a lie and the game tries to kill you with fire.

YP: Portal 2 Cake Jokes