Filling out a W-8BEN form is required to buy US stocks in the app. It is a declaration that you are not a US tax resident.
Dividends issued by US companies will incur a 15% tax at source, rather than the standard 30%.
You can find the W-8BEN form on the Profile page in-app, just head to your Portfolio and click the person icon in the top right-hand corner. You'll see this under 'US Exemption form', if you've already filled this in the option will not be available to you.