The short answer
An integration clause says the written contract is the parties’ complete agreement. Under the parol evidence rule, earlier or simultaneous promises that contradict a fully integrated contract generally cannot be enforced.
Where it fits in your case
When one side relies on a promise made in negotiations that does not appear in the signed document.
What to expect
Earlier promises are usually excluded if the contract is fully integrated.
Later changes are not barred by the rule, though no-oral-modification clauses can complicate them.
Fraud and ambiguity are common exceptions.
What we need from you
Drafts, negotiation emails, and evidence of any promise made outside the contract.
Common misconceptions
“The salesperson promised it, so it counts.” Not if the contract is fully integrated.
“An integration clause blocks every claim.” Fraud claims may survive.