The Torah is so intent that we have no connection to bread for Passover that it prohibits all leavened food stuff to be present or seen in the Passover holiday. So what do we do with all the leftover food stuff and other articles that came into contact or absorbed leavened during the year?
The religious authorities came up with a solution where as the Chametz should be transferred for the duration of Passover to the ownership of a non-Jew. Technically this would mean that the non-Jew owns the items but is holding them on the premises of a Jew. Since it would difficult for every Jew to perform a legal transfer, it is customary that each member of the community sign a document of transfer to the Rabbi who in turn transfers the document with the signature to a non-Jew.
The sale of the leavened, which is done by the Rabbi, is a complicated process which complies with the Torah laws that make it a total sale. The document is redeemed from the non-Jew by the Rabbi immediately after Passover and the various items are returned to their original owners. This transaction is called Mechiras Chametz, the Sale of Chametz.
Chabad of Boulder makes this transaction extremely accessible: all you need to do is click here for your online form.