On Pesach, in addition to the prohibition against eating Chametz (any leavened product), there is also a prohibition against owning Chametz and deriving benefit from it. The Torah therefore mandates that one get rid of all their Chametz before Pesach. The prohibition against owning Chametz is so severe that there is a Rabbinic penalty that any Chametz that was in a Jew’s possession during Pesach cannot be used after the holiday has passed. Whatever one is not able to consume or give away to a non-Jew before Pesach must be destroyed on Erev Pesach by burning it.
Recognizing the significant financial loss that the requirement to get rid of and destroy all of one’s Chametz may impose on Jewish families, the Rabbis came up with a way that the Chametz need not be destroyed. The Chametz that someone owns may be sold to a non-Jew before the holiday begins. Since the Chametz no longer belongs to the Jew, he/she does not transgress any violations if it remains in their home. They are simply storing the Chametz for the non-Jewish owner. After Pesach the non-Jew may sell the Chametz back to the Jewish owner, at which point the Jew may eat and derive all benefit from the Chametz.
The prohibition against owning and deriving benefit from Chametz applies to anything that contains even trace amounts of Chametz. Even non-food items such as medicines and cosmetics may contain Chametz in them.
Pots, pans and utensils that are forbidden for use on Pesach because they have absorbed the taste of Chametz need not be sold. Nor do kitnyot (“legumes”) – food products that Ashkenazi Jews do not eat on Pesach because they may be confused with Chametz, but are not in fact Chametz. Examples of kitnyot include rice, beans, peanuts, etc.
The sale of Chametz is a real, legally binding sale. The non-Jew to whom the Rabbi sells the Chametz is the full owner of the Chametz and must be able to access, and eat the Chametz if he or she so desires. If you will be out of town during Pesach, there must be reasonable accommodations made to allow access to the Chametz if the non-Jewish owner so desires.