Form W-8BEN is a tax form aimed at foreign persons who receive certain types of income in the United States, or from an United States company. The form establishes that the individual is both a foreign person and the beneficial owner of the business in question.
Foreign persons must provide Form W-8BEN to the withholding agent or payer if they are the beneficial owner of the income subject to the tax withholding. You must submit the form regardless of whether you are claiming a reduced withholding.
Individuals must submit the form to the payer or withholding agent prior to receiving income or credits from them. Failure to submit a Form W-8BEN could result in paying a 30% rate or backup withholding rate.
For more information and instructions on how to fill out W-8BEN: https://www.irs.gov/pub/irs-pdf/iw8ben.pdf
See also: Which tax form should I fill out?