Parol evidence rule

Imagine you and your friend make a deal to sell your toy car to them for five dollars. You both shake hands and agree to the deal. Your friend writes down the agreement on a piece of paper and you both sign it. This piece of paper is called a contract. Now, if your friend comes back later and says they actually thought the price was four dollars, you might be confused. You both clearly agreed on five dollars, and the contract says so, too.

The parol evidence rule means that if you have a written contract, you can't go back and use other evidence (like what your friend said later) to change what the contract says. The written contract is the only thing that matters. This helps make sure that people stick to what they agreed to in writing, rather than changing their minds later on. So, if your friend tried to argue that the price was actually four dollars, the parol evidence rule would say that their argument doesn't count, because you both agreed in writing that it was five dollars.