A more thorough explanation:

Definition: The principle of mutuality of estoppel states that a judgment cannot be conclusively in favor of one party unless the opposite decision would also be conclusively against that party.

Example: Let's say that John sues Jane for breach of contract, and the court rules in favor of John. If Jane tries to sue John for the same breach of contract in a different court, John can use the principle of mutuality of estoppel to prevent Jane from relitigating the same issue. In other words, if the court had ruled in favor of Jane in the first case, John would not be able to use the principle of mutuality of estoppel to prevent Jane from suing him again.

Explanation: The principle of mutuality of estoppel ensures that both parties are bound by the same legal decision. It prevents a party from taking advantage of a favorable judgment without also accepting the consequences of an unfavorable judgment. In the example above, if John could use the principle of mutuality of estoppel to prevent Jane from relitigating the same issue, it would be unfair because Jane would not have the same opportunity to do the same if the court had ruled in her favor in the first case.