1040 Estimated tax vouchers
The reason you received these vouchers is because either you are self-employed sole proprietorship or you owed taxes in prior years tax filings.
One reason is to satisfy the projected shortfall in the current tax year and the other reason is to avoid paying penalties and interest. Hence, the IRS are sending you these coupons.
You need to consult with your CPA to ensure that you are withholding the proper amounts of federal income taxes, and if not, use these coupons to make up the potential shortfall.