Not required by law, covers you for your bodily injury bills if you are hit by someone who does not have enough insurance. In the form of a split limit or a single limit with some companies. Example if you purchase $15/$30,000 UM coverage, and you are hit by someone who only has $15,000 of bodily injury coverage but you have $30,000 of bills, the company will pay you up to $15,000 since you were hit by an underinsured motorist.