W-9’s apply only to those living in the US for taxes. They are similar to a W-4 that you fill out if you become an employee for another company. Essentially, W-9’s mean that a company has paid you money but has not withheld taxes on that amount, so come tax season they’ll have to report how much money they gave you, and I think you should get a 1099 form from them as well.
Amounts over $600 you’re supposed to report as money earned and pay taxes on at the end of the year if no taxes have been withheld.