So recently I've seen the term ACE on the form and a few youtube videos for SMB1 and I was wondering what it means and what it does thanks.

I know it stands for arbitrary code execution but I'm not sure about the definition

Nice profile pic, End Cubing. Arbitrary Code Execution, or ACE, is manipulating the arbitrary (random) code of a game to do something specific. It's normally used to finish games much earlier than intended, but its purposes can be crazier as well. Some people have managed to write games into other games using the empty space in the code. Hopefully this was somewhat helpful. Cheers.

Adding onto that, ACE only works in games where there are exploits to run bytes from arbitrary memory adresses as code. There also needs to be a way to control the writes to memory and be able to write the right bytes to create an actual instruction to run. SMB1 ACE is incredibly unlikely, since it's such a simple game.