FMM is an acronym for Forma Migratoria Multiple or Multiple Immigration Form. This link will take you to the Instituto Nacional de Migración online FMM form. After completion, the site will send you to a bank site to pay then provide an opportunity to print. The form must be presented at Mexican immigration to be stamped then it is value for your stay. The completed and stamped FMM is required for any tourist visit to Mexico, much like a Tourist VISA in many other countries.