Integration refers to the concept that the agreement you sign represents the entire agreement, and verbal promises made prior to that are not part of the agreement (for example, separate promises about signing bonus, moving allowance, etc.).
There are often clauses that specify that “this is the entire agreement…” but even without such language the integration concept generally applies.
There is an exception to the integration concept if the original promises made to the employee, prior to provision of the contract, were made with the explicit intention of not honoring them. In such cases the other party—the employer—has acted fraudulently. But it’s very difficult to prove such intent.
In some states, if the employer fails to reveal material information when it offers you a contract, for example, that the practice is currently under regulatory investigation, you may have grounds for legal action. This may be the case even in the absence of a misleading clause in the agreement explicitly stating, “we are not under investigation.”