An integration clause is a provision of a contract which states that the parties intend for the written contract to be a final, complete, and exclusive statement of the agreement. Without such a provision, there is no assumption that the written contract is the complete or exclusive expression of the agreement.
In practice, when parties put together a sales contract they engage in discussions and negotiations regarding how the interaction will take place. What the parties discuss before putting the agreement in writing, and even what they discuss at the same time the agreement is put in writing, essentially become irrelevant to the extent the written contract itself specifies the terms that were the subject of the discussions. Once specific terms are put into writing and agreed to by the parties, those terms govern regardless of what might have been discussed or put in writing otherwise.
However, parties need to understand that the terms of a contract only govern the issues they actually address. There is no obligation for parties to put all of the terms they might agree to into writing. They are free to only list as much, or as little, as they want. As a result, there is no presumption that the writing covers everything.
The inclusion of an integration clause in a contract does a lot to clarify and limit the agreement in place. The clause works to exclude from the actual agreement anything outside of the writing, whether or not the parties discussed those other issues.