CP 14 shows the underpaid tax according to IRS records. The notice shows the tax reported on the return, the payments the IRS applied to your account, and the remaining underpayment they have calculated.
Generally, this is caused by a misapplied or missing payment. Check the payments as outlined on the notice the IRS has applied to your account and compare them to your records. If the IRS has missed a payment you made, you will need to provide information to document the payment so they can locate the payment and properly credit your account.
Additional interest and penalties may accrue if the balance is not paid in full by the due date. If you do not agree with the additional tax due, a response should be made to the IRS as soon as possible to settle the issue with your account.
Sections of the Notice:
Sample CP 14: PDF Pages 1-4
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.