I just bought the HD version and i noticed this on the save menu but i've no idea what it does. Is it something i enter on a website for some in-game bonus?

If I recall correctly there were cheats on some website at the time, needless to say they are long gone. Nothing fancy, just your basic cheats. It's the same system used in the LEGO games.

You can enter the code at the website (beyondgoodevil.com, go to "Access the darkroom") and if your "score" is high enough, you get to play some hacking minigame thing. If you beat it you get the combination to the cabinet next to the table where you play the disk game against Francis. It has the last M-Disk where you can access a 2 player version of the disk game.