Zorgverzekeringswet! Bless you.
The zorgverzekeringswet (loosely translated to health insurance law) is meant to make working people pay an extra tax to their health insurance via the belastingdienst.
The zorgverzekeringswet is a solidarity law that helps insurance companies pay for medical costs: every tax-payer contributes a small amount, which is usually automatically deducted from your salary, depending on your income. For employees, the tax is 6.70% of your income, to a maximum of €57,232.
If you are self-employed, you need to calculate the amount yourself. In 2020, the percentage of health insurance tax you need to pay over profits is 5.45%, over a maximum profit of €57,232 per year. If your profit is €57,232 or higher, you need to pay €3,119.
However, if the tax authorities receive too much health insurance tax from you, they will refund the excess.
For more detailed information, check out this article on the tax authority website.