Uninsured/underinsured motorist coverage (UM/UIM) is part of your own auto policy. It steps in when the at-fault driver carries no insurance (UM), too little insurance to cover your losses (UIM), or cannot be identified at all — as in many hit-and-run crashes.
It matters because a meaningful share of California drivers are uninsured or carry only minimum limits, which serious injuries exhaust quickly. UM/UIM is often the only realistic source of recovery in those crashes.
Check your policy's declarations page for "UM/UIM" limits. Claiming under your own policy is a contractual right you paid premiums for; insurers may still dispute value, so document the crash and injuries the same way you would against another driver's insurer.