In a personal auto policy, what does 'uninsured/underinsured motorist (UM/UIM)' coverage protect against?

UNINSURED/UNDERINSURED MOTORIST (UM/UIM) COVERAGE: Protects the INSURED when injured by an at-fault driver who: has NO insurance (UNINSURED motorist — UM); or has INSUFFICIENT insurance to cover the full loss (UNDERINSURED motorist — UIM). COVERS: The insured's (and passengers') bodily injury, and in some states property damage (UMPD); steps in where the at-fault party can't pay; FIRST-PARTY coverage (the insured's own policy pays); WHY IMPORTANT: Many drivers are uninsured or carry only minimum limits; UM/UIM ensures the insured isn't left uncompensated by an at-fault driver who can't pay; SOME STATES require it or require it be offered; PART OF auto policy structure (along with liability, collision, comprehensive, medical payments/PIP); UM/UIM is a key personal auto coverage on the casualty exam — it protects the insured against the gap created by at-fault drivers with no or inadequate insurance.