What does it mean to apply estimated tax to 2020 refund

[John105]

The government keeps the amount you apply and it is a withholding for your current year's taxes.

[VolvoGirl]

An overpayment is your refund.   You can have them keep all or part of your refund as an estimated payment towards next year's tax return.  But most people want to get their refund now.   So to get your refund now do not say to apply it to next year.