It is a tax identification number that you use when paying taxes in your country. In the rare event you do not have one you may put N/A.
For US individuals, it is your Social Security Number. For US Companies, it is the Tax Identification Number.
THE USE OF TAX FORM
The W9 form is for US Individuals and Companies
The W8-BEN form is for International Individuals
The W8-BEN-E is for International Companies
Link to IRS Website to search:
Your certificate of ownership will be processed within one month after receipt of your paperwork, including a completed w-9 or w-8BEN form