If you had an underpayment of tax in a previous year HMRC may have included this in your 2013/14 tax code. If this was the case the underpayment would be noted on your P2 coding notice and the amount of the underpayment is stated in the notice on this notice.
You can also check the amount of any underpayment through your Government Gateway login online.
If you had an underpayment included in your tax code during 2013/14 then include the amount in this box.