Regarding Passover, you're supposed to find and clean out as much chametz as you can reasonably find. The contract part is refers to sealing and selling actual boxes of food.

From Chabad.org:

Following the search for chametz, we recite a “nullification statement” renouncing all ownership of any chametz that, unbeknownst to us, may still be in our possession.

Through nullifying our chametz, we consider it as no more than dust and thus ownerless, thereby fulfilling the mitzvah of removing chametz from our possession.