It is a certificate an insurance company files with the Nebraska DMV confirming that a liability policy meeting the state minimum is in force for a named driver. It is proof of coverage, not a type of coverage, and only an insurer can file it.
More detail
The form exists so the state can verify, continuously rather than once, that a driver it has flagged is carrying liability insurance. Nebraska requires it under the Motor Vehicle Safety Responsibility Act in chapter 60 of the state statutes. Three consequences follow from the fact that the insurer files it. First, a driver cannot file one directly; there is no counter at the DMV where an individual submits a certificate. Second, a policy from a carrier that does not handle filings cannot satisfy the requirement, no matter how good the policy is. Third, the insurer is obligated to tell the state if the policy lapses, which is what turns a missed payment during a proof period into a licensing problem rather than only a coverage problem.