The IRS has made changes to your tax return and there is nothing owed or no refund based on the adjustment (CP 13A involves changes to the Earned Income Credit).
The notice explains the changes to the tax return (and/or Earned Income Credit).
If you agree with the adjustment(s) make the appropriate changes to your tax return record. If you do not agree with the change, call or write to the IRS at the phone number or address given in the header section of the notice.
Sections of the Notice:
Sample CP 13:Sample CP 13A: