Non-resident tax is the result of owning US assets in your portfolio. When US assets pay out dividends, they are subject to a tax for non-residents in TFSA, RESP accounts and non-registered accounts. This is because these accounts are not recognized by the US, while the RRSP is recognized through a tax treaty.
Despite this tax, there is no requirement to file any forms with the IRS. It is also still very advantageous to own US assets, despite this withholding tax. Owning US assets helps to diversify the holdings of your portfolio and generally make up a notable portion of Wealthsimple portfolios.