Underinsured motorist coverage is part of a car insurance policy that helps pay for your medical bills if you're hit by a driver who doesn't have enough liability car insurance to cover your medical bills. Typically, when you're in an accident and the other driver is at fault, their auto liability coverage would help pay for your medical bills/injury and/or repairs to your car. But if the at-fault driver's insurance can not cover all of your costs because they don't have enough limits, you may have to pay out of your own pocket for those expenses. That's where Underinsured coverage may help.