The N-400 form is an application used by lawful permanent residents in the United States to apply for U.S. citizenship. This form is a crucial step in the naturalization process, allowing applicants to demonstrate their eligibility based on residency, good moral character, and knowledge of U.S. history and government. Filing the N-400 form involves providing personal information, including details about residency, employment, and family. Applicants must also pay a fee and may need to attend an interview with a U.S. Citizenship and Immigration Services (USCIS) officer to discuss their application and take a civics test.