Uninsured motorist coverage is part of nearly every auto policy in Missouri and Illinois. It steps in when the at-fault driver has no liability insurance. Either because they were uninsured at the time of the crash or because their identity is unknown (a hit-and-run, in some cases).
UM benefits come from your own policy, but the insurer treats the claim adversarially. You're effectively in a contract dispute with your own carrier over what they owe you. Most UM cases involve negotiating with your insurer the way you would a third-party insurer.
Missouri does not merely require insurers to offer UM coverage, it requires the coverage itself: RSMo § 379.203 puts it in every auto liability policy at the bodily-injury limits set by RSMo § 303.030. That mandate is what drives Missouri stacking law. Because UM is statutorily required, anti-stacking clauses are void against public policy to the extent they would deny the required minimum for each insured vehicle, so UM limits can often be combined across vehicles on the same policy. Underinsured motorist coverage is different: Missouri does not mandate it, so it is governed by the contract and a clearly written anti-stacking clause generally holds.