A Social Security card is an official document issued by the U.S. government that contains a unique nine-digit number assigned to individuals. This number is primarily used to track earnings and determine eligibility for Social Security benefits, such as retirement and disability payments. The card is essential for various purposes, including applying for jobs, opening bank accounts, and filing taxes. It is important to keep the Social Security card safe, as it helps protect against identity theft and ensures that individuals can access their benefits when needed.