Do you know what U.M. stands for?
OK, if you're trying to settle a car wreck case on your own. If you're talking to the insurance adjuster and they talk about UM; what that means is its Uninsured Motorist Coverage.
In Colorado that means you bought insurance that makes up the difference between what the person that hits you has and what your case is ultimately worth.
So, UM stands for uninsured motorist coverage.