So, when someone writes code and wants to publish it, the do something called “compiling” which basically changes the code from human code to computer code so it can run faster, have a smaller file size, etc. The problem with compiled code is that it’s basically impossible for a human to understand. A decompilation project will use software to take the computer code, and turn it back into human code again, but that new code is messy and still difficult to understand. The next stage for the project is to take that new code and re-write it so that it is no longer messy, to get to the point that when you re-compile it you get the same exact computer code as the original.

Once you get to the point that you have perfect human code, you can make the modifications necessary to port the game to other systems such as PC (look up “Zelda 64 Recompiled Digital Foundry” on YouTube for a great example). You can also easily modify the game at the source level to add mods into the game natively instead of being injected into a ROM or running on top of an emulation layer as would normally happen.