Social Security Disability Insurance (SSDI) is a federal program that provides financial assistance to individuals who are unable to work due to a qualifying disability. To be eligible, applicants must have a work history and have paid into the Social Security system through payroll taxes. The program aims to support those who have become disabled and cannot earn a living. SSDI benefits are based on the applicant's average lifetime earnings and can help cover essential expenses such as housing, food, and medical care. The application process can be complex, often requiring medical documentation to prove the severity of the disability.