A more thorough explanation:

In contract disputes, parol evidence refers to any agreement that is not included in the written contract. The parol evidence rule states that these agreements made outside of the contract are not admissible in court, unless there is evidence of fraud, duress, or a mutual mistake. The rule is in place to prevent dishonest attacks on contracts.

The parol evidence rule prohibits extrinsic evidence, including prior or contemporaneous oral agreements and prior or contemporaneous written agreements, that contradict or create a variation of a term in writing that the parties intended to be completely integrated. This means that any information leading up to or during a contract that is not included in writing is considered inadmissible evidence and is excluded from the jury. The jury will only look at the writing within the document itself to decide a contract dispute.

There are two exceptions to the parol evidence rule: the collateral contract exception and the ambiguity exception.

The collateral contract exception applies when the extrinsic agreement is a collateral one, meaning it is not distinct and independent from the original written agreement. The extrinsic agreement must not contradict the express or implied provisions of the written contract, and it must be one that the parties would not ordinarily be expected to embody in the writing.

For example, in Mitchill v. Lath, the plaintiff purchased land from the defendant because the defendant had allegedly promised to remove the ice house. The defendant did not remove the ice house before or after the plaintiff’s purchase. The court enforced the rule of parol evidence and barred the extrinsic agreement because the court found that the removal of the ice house could be reasonably expected to be included in the original written contract.

The ambiguity exception applies when the language in the original written contract is reasonably susceptible to more than one meaning. The court may permit the admissibility of parol evidence to determine the meaning of the contract language under the ambiguity exception.

For example, if a contract states that a buyer will purchase a "red car," but there are multiple shades of red, the court may allow parol evidence to clarify which shade of red was intended.

Overall, the parol evidence rule is in place to ensure that contracts are upheld and that parties cannot use outside agreements to contradict the terms of a written contract.