Chametz is any food product made from of wheat, barley, rye, oats, spelt or its derivatives. During Passover it’s a mitzvah to get rid of chametz, as it says, “You shall eat nothing leavened, in all your settlements you shall eat unleavened bread.” (Ex. 12:20)
In addition to not eating chametz, or benefiting from chametz on Passover, Jews are also not permitted to OWN any chametz during Passover.
Mechiras Chametz, Chametz Sale, is a binding and legal sale of chametz that can be performed before the holiday to a non-Jew. We promise to buy it back after the holiday. While this seems like a loophole, it’s a real and binding sale, and ensures we celebrate Passover properly! (The sale is a complex legal procedure that you should not try at home.)