It's basically a flexible way to mod the game. Actually they're just mods, but not in a monolithic way, it's very modular ...

http://wiki.beyondunreal.com/UE3:Mutator_(UDK)

What's a mutator? It's a tiny bit of code which is designed to modify an Unreal Tournament match in some way, perhaps by lowering the gravity, making players fatter as they accumulate frags or even remove all the powerups from the game. The effects of mutators are temporary, and unless you explicitly specify to keep them permanent you will have to re-set your mutator preferences for every game you want to play.

For such a simple concept, mutators add enormous replayability to Unreal Tournament, by allowing endless original combinations of maps, bots, mutators and more. Even if you're a purist you doesn't normally bother to stray too far from the path of hardcore DeathMatch or CTF, everyone should try these out at least once just for the fun of it.

http://planetunreal.gamespy.com/View.php?view=UTGameInfo.Detail&id=20

^3Pikachu: no, we do not plan something like this for the first release. However, we would like to add support for bigger mods than only cvar settings, but we will think about it after release.