A more thorough explanation:

The compulsory-counterclaim rule is a legal requirement that a defendant must present any counterclaim that arises from the same transaction or occurrence that is the basis of the plaintiff's claim. This rule is outlined in the Federal Rules of Civil Procedure, specifically Rule 13(a).

For example, if a plaintiff sues a defendant for breach of contract, and the defendant has a counterclaim for unpaid invoices related to the same contract, the defendant must include that counterclaim in their response to the plaintiff's lawsuit. Failure to do so may result in the defendant being estopped, or prevented, from asserting that claim in the future.

The purpose of the compulsory-counterclaim rule is to promote judicial efficiency and prevent multiple lawsuits arising from the same transaction or occurrence. By requiring all related claims to be brought in the same lawsuit, the court can resolve all disputes between the parties in one proceeding.