Estimated tax is the method used to pay tax on income that is not subject to withholding made directly by you towards the IRS. These payments can be made only during the tax year from January 1 until December 31 and you must have used form 1040-ES when making these payments.
The estimated tax payments do not include tax withheld by your payer on your US income earned during the year and do not include liability (balance due) you have paid on your last year's Federal income tax return.