Is for the full amount you paid? When did you pay it? If it was after July 15 it might be for interest. Or if you owed too much it might be a penalty for not paying in enough during the year.
How did you pay it? Did you send in a check? Did you use the 1040V voucher? If you used a 1040ES slip that was to make an estimated prepayment for NEXT year 2020 return.
