We're excited to announce that we've fully migrated the Xbox Forums to their new home, here on the Microsoft Community forums!
As of today, going to forums.xbox.com will redirect you here to the Xbox area of the Community forums. We've moved all of your existing threads over so you can continue and start conversations with the new forums experience.
We sincerely appreciate the feedback you have provided over this transition and can't wait for you to check out the new Xbox Forums!
It is an option that changes the rules of the game, to make it simple to understand.
Somechanges are cosmetic, like Bighead mode, some changes affect gameplay, like giving Unlimited Ammo, etc. They require being unlocked in Gears of War 3 as far as I know.
Made most famous in Unreal Tournament, a trademark of Epics if you will, they add some variety and spice to multiplayer if implemented right, and in general can just be damn outright fun.
I'm sure googling Unreal Tournament Mutators or Gears of War 3 Mutators will give you a much better explanation and idea of what they are than I could probably explain however. But you should get the idea. :)