The IRS has adjusted your tax return which affected the amount of the refund applied to the following year's estimated tax payments as requested on the tax return.
The notice explains the changes to the tax return and the resulting adjustment to the amount applied to the estimated payments, you should adjust your tax return and estimated payment records to match the IRS records if you agree with the change.
Sections of the Notice:
Sample CP 10: PDF Pages 1-2
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.