A more thorough explanation:

A declaration of estimated tax is a form that certain individuals and businesses must file with the IRS to report their current estimated tax owed. This form is accompanied by periodic payments of that amount. The purpose of this requirement is to ensure that taxes are collected from taxpayers whose incomes are not fully taxed by payroll withholding, such as self-employed individuals.

• John is a freelance graphic designer who does not have taxes withheld from his income. He must file a declaration of estimated tax and make quarterly payments to the IRS.
• Sarah is a small business owner who pays herself a salary but also receives income from the business that is not subject to withholding. She must also file a declaration of estimated tax and make quarterly payments.

These examples illustrate how individuals and businesses who do not have taxes withheld from their income must file a declaration of estimated tax and make periodic payments to the IRS. This ensures that taxes are collected throughout the year rather than in one lump sum at tax time.