Dividend withholding tax is the tax a company must take off a dividend before the payment is made to the shareholder. This is then passed onto the government in which the share is domiciled.
The tax rate will depend on where the share is registered. For example, UK share dividends aren’t taxed until they reach £5000 per individual, after which you’d be taxed at the relevant income tax rate. However, dividends on US shares are taxed at 30%.