Under the Immigration and Nationality Act (INA), a visa applicant must meet the required conditions to be qualified to obtain a visa. When one or more requirements can not be met, the applicant is ineligible for the visa which will cause a denial. In certain circumstances, the applicant may request a Waiver of Ineligibility for a particular reason.
For example, a U.S. citizen petitioner can only file for a K1 visa twice and can not file the second one until two years after the first one was approved. In any situation you want to apply for a third K1 visa or apply for the second one within two years, you must file a K1 fiancé visa waiver for multiple filings.
All waivers of ineligibility will be submitted to the Department of Homeland Security (DHS). The Attorney General will review your situation and determine whether a waiver should be issued.