Estimated tax payments
Tax you send the IRS yourself during the year, usually in four payments, on income that has no tax withheld, like freelance or gig pay.
Employers withhold tax from paychecks. When you are self-employed or have other income with nothing withheld, you pay estimated tax instead, usually by April 15, June 15, September 15 and January 15 of the next year.
The payments cover income tax and self-employment tax. If you expect to owe $1,000 or more and don't pay enough during the year, you can owe an underpayment penalty, figured like interest.
Example
Last year's total tax was $8,400 and nothing is withheld this year: $8,400 ÷ 4 = $2,100 per quarterly payment.