A more thorough explanation:

Qui tam is a Latin phrase that means "who sues on behalf of the King as well as for himself." In a qui tam action, a person called a relator brings a lawsuit against a person or company on behalf of the government. The government is considered the plaintiff, not the relator. If the government wins the case, the relator who brought the lawsuit receives a share of the award.

For example, the federal False Claims Act allows qui tam actions against people or companies who have defrauded the government. If a relator wins a False Claims Act qui tam action, they can receive up to 30% of the government's award. In the case of United States ex rel Eisenstein v. City of New York, the Supreme Court ruled that a relator could bring a qui tam action against a city for falsely claiming to have provided mental health services to Medicaid patients.

Qui tam actions are important because they allow individuals to help the government fight fraud and corruption. They also provide an incentive for people to come forward with information about wrongdoing, as they may receive a portion of any money recovered by the government.