This one needs context.
So basically: After some digging, someone in the Five Nights at Freddy's thread posted an in-editor shot of the room you play from. From this rare perspective we were able to see the person in the player's seat.
Except, of course, there was no one there, because why would anyone model a character you'll never see? Instead there was just a green camera box.

So the joke was that the true protagonist was a green cube. And we all know someone who fits that description.