Line 38 is the estimated tax penalty from Form 2210, added to the amount you owe. You can leave it blank and let the IRS figure it.
Line 38 sits under "Amount you owe" on page two of Form 1040, right after line 37. It holds the penalty for paying too little tax during the year through withholding and estimated payments. The figure comes from Form 2210, line 19, and it is part of the amount you send with the return.
You do not have to fill it in. The instructions say the IRS will figure the penalty if you leave line 38 blank, and the bill arrives as a CP30 notice a few weeks after processing. The IRS treats each quarter's required installment as equal and treats withholding as paid in four equal parts. That figure is too high when your income or withholding came late in the year.
Fill it in yourself when a Form 2210 adjustment lowers the penalty. The most common is the annualized income method for income that came late in the year. If your withholding was uneven, box D on the form applies it on the dates it was actually withheld. In either case attach Form 2210 and put its line 19 on line 38.
An amount on line 38 does not mean the return is wrong. It means the payments during the year fell short of the safe harbor. Paying a quarter of that number on each due date next year keeps the line at zero.
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Sources: Form 1040 instructions· Form 2210 instructions