Answer
When you invest in financial instruments, you might encounter taxation on your income, such as on dividends or coupons. Directly at the pay-out of the income, tax is withheld and paid to the tax authorities of the resident country of the paying organisation (source country). This form of taxation is called withholding tax. Examples are dividend withholding tax and interest withholding tax.
Withholding tax is usually withheld by the paying agent of the paying organisation, but withholding can also take place higher up the custody chain. DEGIRO usually receives the income as a net amount, after the deduction of the withholding tax.
Withholding tax rates differ depending on the source country and the nature of the income paid.