The W-8BEN form is a US tax document issued by the US government. If you don't live in the United States and you want to trade US stocks on Bitso, you need to fill it out to declare your tax residency in your own country.

This form keeps you from paying tax twice on the same gains, once in the United States and once in your country. It also lowers the tax withheld on dividends you receive from your US market investments.