Protects you if you're hit by a driver with no insurance or not enough coverage to pay for your damages.

Uninsured motorist coverage, often shortened to UM, pays for your injuries and sometimes property damage when the at-fault driver has no insurance or cannot be identified. Examples include hit-and-run crashes or accidents with drivers who never carried liability insurance. Underinsured motorist coverage, UIM, applies when the at-fault driver has insurance but their policy limits are too low to cover your medical bills, lost wages or other compensable damages.
UM and UIM are for any driver who wants to avoid assuming the other driver’s financial responsibility. That includes people who frequently drive commuters, families with children and anyone with assets to protect.