A more thorough explanation:

The parol evidence rule is a legal principle that states that any agreement made outside of a written contract is inadmissible in court unless there is evidence of fraud, duress, or a mutual mistake. 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 therefore only look at the writing within the document itself to decide a contract dispute.

For example, if two parties sign a contract to buy a car and later have a verbal agreement to change the payment terms, that verbal agreement cannot be used in court to dispute the original contract.

The parol evidence rule is codified in U.C.C. § 2-202, which states that terms in a writing intended by the parties as a final expression of their agreement may not be contradicted by evidence of any prior agreement or of a contemporaneous oral agreement but may be explained or supplemented by course of dealing, usage of trade, or by course of performance; and by evidence of consistent additional terms unless the court finds the writing to have been intended also as a complete and exclusive statement of the terms of the agreement.

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, does not contradict the express or implied provisions of the written contract, and is one that the parties would not ordinarily be expected to embody in the writing.

For example, if two parties sign a contract to buy a car and later have a verbal agreement to include a free oil change, that verbal agreement may be admissible in court if it meets the three conditions of the collateral contract exception.

The ambiguity exception applies when the language in the original written contract is reasonably susceptible to more than one meaning. In this case, the court will permit the admissibility of parol evidence to determine the meaning of the contract language.

For example, if two parties sign a contract to buy a car and the contract states that the car will be delivered "as is," but there is a dispute over what "as is" means, the court may allow parol evidence to determine the parties' true intentions.