Its just a number assigned to every American person usually at birth nowadays. It is assigned the the Social Security Administration. It was designed as a Social Security account number, but is also used as a Tax Identification Number by the IRS. And a lot of places treat it like it is some super secret password, but it is just a number, and it is a number that everyone wants, and everyone and their dog- including multiple government agencies- has made publicly available, so now everyone and their dog and their dogs cousins dog has access to them, so it makes absolutely no sense why anyone would treat them as secret.