The TIN (taxpayer Identification Number) can either be your Social Security Number or it can be the Individual Taxpayer Identification Number (ITIN).
An ITIN is a 9-digit number issued by the U.S. Internal Revenue Service (IRS) to individuals who are required for U.S. federal tax purposes to have a U.S. taxpayer identification number but who do not have and are not eligible to get a Social Security number (SSN)